How precise do spoke sizes need to be?

A typical spoke has around 8-10 mm of threading on it, and you don't necessarily have to engage all of it with the nipple. So it's not super sensitive to small variations in length.

For most folks, too long or two short really doesn't matter as long as it's within about 2 mm, so just round to whichever stock size is closest to ideal.

If you go beyond the 2 mm, it is better to be a little short than long. A spoke has quite a bit of thread on it, and you don't need all of the nipple engaged to the thread (though more is better), so you have several mm of room to work with. However, if the spoke is too long, and the spoke pokes out of the nipple, that may be trouble as the rim strip may not be able to protect the tube from the spoke. Additionally, you may bottom out the spoke nipple, and be unable to tension any further.

So if I were in your position, while the 292 would be your best choice, I'd be tempted to just get all 295 so I don't have to keep track of the two different sizes if I had a good double wall rim, but it's not a ton of work either to just have two different stacks.

A spoke of ideal length is such that it is flush with the head in a finished wheel. Then it engages all threads, maximising stability. However, from there you "only" have 1.5mm tolerance towards too long [for DT spokes and DT nipples, measured myself some time ago], then all the threads of the spoke is wound through the nipple and it binds, you cannot tension more. This is bad, insufficiently tensioned wheels are bound to fail.

For this reason, spoke lengths are calculated with respect to the bottom of the slot in the head of the nipple, that is 1mm below to top of the head [DT standard]. So from there you get 2.5mm tolerance toward too long... if the calculator has the correct length. Try it, ask 5 different calculators and you will get 3 different answers.

Too short is risky as well, but less easy to quantify. The threads in your average nipple are 4mm long, engaging just 3mm is ok, 2mm might also... That would be 1mm short of the ideal length. If you use aluminium nipples, however, 2mm will most likely not suffice.

A frequent advise given is to round up to the next even number unless the difference to the lower even number is 0.5mm or less. So with 292.4 get 292mm spokes, with 292.8mm get 294mm.

In essence, you will want to be pretty precise. 2mm too long might just be ok, 2mm short might be too short. Be sure to measure the ERD yourself (!) and as precise as you are able to.

This topic is really poorly understood. Anyone who ever who tells you it's fine to just round up or round off when choosing spoke lengths hasn't been through the disaster scenarios that can be caused by it, or perhaps doesn't remember because the experience is too painful and embarrassing. Rounding up can cause you to run out of threaded length before the spokes are fully tensioned. Always round down unless you can test and measure the spokes in question before building to avoid running out of threaded length or poking the tube. If you have to order spokes, just round down. If your calculated optimal length is accurate (and there's no reason it shouldn't be) and you must round down to the next even number from there (as in the 292 in your example), nothing meaningful will ever be lost in terms of durability from that extra 1.99mm max.

What gets overlooked in this discussion is that there are whole brands/models of spoke, or spoke/nipple combinations, that offer zero ability for the spoke to protrude past the top of the nipple. They don't have the threaded length to do it. So rounding up walks you right into the failure situation of running out of thread and having to either start over or accept an untrueable wheel with bad tension. Various Wheelsmith bags I've used, for example, have been like this, with Wheelsmith nipples.

Contemporary DT Champions and Competitions, the most commonly encountered premium spokes, avoid this occurrence by having some superfluous/generous threaded length. Some other spoke brands and models do not, and give you no leeway for the spoke to be long. And single wall builds naturally have no such leeway.

Some point out that having aluminum nipples miss more than 1mm or so of engagement introduces some risk of the nipple breaking. That's probably true and I recommend shooting for the best possible thread engagement on aluminum nipples, which for practical purposes on conventional nipples is between the base of the screwdriver slot and the top. However, you'll never know if that's what caused it, because al nipples break anyway, especially for 2mm spoke ends. (Al nipples for 1.8s are far less prone to breaking because of the extra material.)

Your calculation must be accurate. You don't want to miss more thread engagement than around 2mm from perfect. Never use a calculator where you can't verify that the math it's doing is the pure classic spoke length formula. Some calculators try to invisibly factor in estimates of spoke stretch and rim compression, which you should do manually based on the parts you're using. (Spoke gauges differ in stretch, the amount of tension also causes it to differ, and the stoutness/cross-sectional area of rims make them differ in compression.) Because it's an Excel spreadsheet and all its formulas are right out in the open, Spocalc is my preference. And always measure the hub and rim yourself. It seems pedantic, but I promise that if you build enough wheels, or are ever working for a wheel producer and are tasked with doing spoke calculations and there's never time to do test runs of anything (this was my situation that taught me good spoke calculation habits), you'll find all this necessary to have consistently good results.

If you have to order the hub/rim along with the spokes (particularly the rim, because manufacturer list measurements for hubs are usually very accurate), just take vendor/manufacturer listed measurements on faith for calculation purposes, but measure yourself once you do have everything to make sure what you're about to do is going to work. Second-guess published ERDs early and often.

Wheelbuilding texts almost never remember to tell you that longer length nipples usually have more threads, and you need to compensate for this to avoid bottoming out the threads on the spoke. Most handbuilt wheels are done with 12mm nipples, but if you ever use 14s or especially 16s, you'll find the rim's ERD is effectively a few millimeters smaller. Or, in other words, ERD is usually calculated as the diameter of the circle formed by the tip of the spokes in a state of optimal thread engagement. Usually that's synonymous with the tip of the spoke being just about flush with the top of the nipple, somewhere between the bottom of the screwdriver slot and the top. 12mm nipples can all handle that, but most of the time if you thread an aftermarket/handbuilding-worthy/premium (however you want to say it) spoke into a 16, you'll find it runs out of thread and bottoms out before it reaches the top.

If you do need to round up, or insist on getting full thread engagement, always test it out first by taking a nipple and spoke that you're about to use and see how far the nipple screws on before it bottoms out, and then measure the amount of protruding spoke you have to work with to make sure it will be acceptable with whatever your calculated length was. Also make sure the rim's depth is such that it won't cause spoke to protrude up and poke the rim strip/tube. If the spoke meets these criteria it's fine, but it can't just be assumed that it will until you test. Do that test on enough spokes and you'll see.

A practical experience of mine

The spokes on my last build had 10mm thread, and the nipples, despite being 12mm long, only had 7mm thread. If, for example, I had used 3mm longer spokes (which I did, because I had them laying around), there would have still been full 7mm thread engagement ✓; because of the longer thread on the spoke. If, on the other hand, I had used 3mm shorter spokes, there would have only been 4mm thread engagement; perhaps not safe enough.

With lower spoke count, thread engagement is more important, because stress is distributed among fewer spokes.

Trick: You can use nipple washers to effectively increase the ERD, better suiting longer spokes. Regular washers may work too, if proper in size.

Some informative resources, from more experienced wheel builders, regarding this topic

Here is a video from Bill Mould (a very experienced wheel builder, and scientist) explaining spoke length, nipple threads, and more:

Video summary:

Spoke threads are rolled instead of cut; material is not removed. The spoke is thinner between its threads. Longer nipples don't have more threads, so it's not a good solution for too short spokes. If the spoke protrudes a little bit over the nipple that's fine. Too little thread engagement leaves the aluminium nipple head unsupported, which can snap off.

Jobst Brandt also mentions in his book, "Bicycle Wheel":

Nipples are made in various lengths to suit different rim thicknesses. Many wooden rims, for instance, required 25 mm long nipples to reach through the rim. Nipples must be long enough so that they can be turned by a spoke wrench. Regardless of its length, a nipple usually has no more than 20 threads at its head end and a smooth bore for the remainder of its length.

Here is another video from Bill Mould:

Summary:

Leaving unengaged threads on the spoke creates a weak point in the spoke. Forcing the nipple past the threads of a longer spoke will strip/flatten some of the nipple threads because nipple material is softer.

And his most recent video about optimal spoke length:

Summary:

There is a weak point in the spoke (where the threads start) even when the threads are fully engaged. You can eliminate the weakness by forcing the nipple past the spoke's threads. Which he considers the best scenario, with the longest spoke life.

Hopefully now you can decide on your own what's the best spoke length, and if shorter or longer is better in your particular case.

1. How precise do the spoke lengths need to be. If they are out by a millimetre is it likely to cause a significant problem?

No. A millimeter too long is fine, if the effective rim diameter (ERD) is measured very accurately (but it may not be measured accurately enough). I measured that DT Swiss Pro Head Standard 2mm diameter 12mm long nipples allow the spoke to be 1.4mm too long, if longer then the nipple threads touch the unthreaded spoke portion and tightening requires lots of force to deform the nipple threads. I also measured that the wrench flats are 1.5mm deep and presumably it won't hurt if the spoke threads stop where the wrench flats start. Thus, a millimeter too short is fine, too. So, if you have an accurate ERD, you can safely round up by millimeter, or round down by 1.5 millimeters on double wall rims where the spokes do not touch the rim tape.

If you have a single wall rim where the spokes directly touch the rim tape, they should preferably not protrude much above the nipples. Thus, round up at most 0.1 or 0.2 millimeters and the rest goes rounding down.

The spoke manufacturers or retailers have realized this. I am unable to find DT Swiss Alpine III spokes in 1mm accuracy. There is 2mm difference between the neighboring sizes. I'm not sure if this is due to DT manufacturing only in 2mm accuracy or the retailer only stocking in 2mm accuracy.

Note that spokes stretch when tensioned. For example, DT Swiss Alpine III spokes require tensioning by 1.5 turns and stretch 0.65 mm to obtain 1200 N tension. If the rim ERD measurement does not take into account the fact that spokes are tensioned, your spokes will be slightly too long. This won't hurt unless you rounded up by more than 0.4mm-0.5mm or so.

You can also forcefully turn the nipples past the spoke threads. It can be argued that it's better than using too short spokes: the nipples are less likely to fracture due to threads being exposed, and it works like thread glue: the nipples will stay tight even if you failed to tension all spokes enough.

If they are out by 1-2mms, is it better if they are too long or too short. So if it turns out to be impossible to get a 293mm, would a 294mm or a 292mm be a better alternative.

Depends on whether you have a single wall or a double wall rim.

On a single wall rim, there is heavy preference towards having too short spokes so that the spokes don't have a sharp pointed end that could puncture the inner tube through the thin rim tape. You should only round 0.1 or 0.2mm up, the rest goes rounding down.

On a double wall rim, you can round to the nearest size. There is about as much freedom in rounding down than there is in rounding up.