What is the ideal hydration for bread dough?

There is no "proper" ratio. 55% is indeed a very common ratio, but by no means the only possible. Here is a somewhat extreme example: 100% hydration. On the other hand, you can also go very low, if you add oil and use a low-gluten flour. I've made 35% more or less successfully.

The hydration is very dependent on your flour. A low-gluten flour will fall apart at high hydration. You must use bread flour or durum flour for high hydrations, else you'll get a slurry instead of a dough. Gluten development is, however, hard to achieve with runny dough. This is why some argue for double hydration: take a look at this breadcetera post. (Disclaimer: I haven't tried that one myself).

If you want to stick to easy-to-make bread, 60% is the target for classic French bread and very easy to work it. The farther away your hydration gets from 60%, the harder to work with the dough. If you are new to bread, aim for recipes closer to 60% until you get comfortable with your skills.

The above post assumes yeast wheat bread. Other grains and sourdough have different handling properties.

There is no proper ratio, it really depends if you want a light bread or one that is more full. You can reach even >100% hydration, but there mixing by hand may be a bit tricky.

70% hydration is a medium-high ratio: you should mix until you start having a stringy textrure in the dough, due to the formation of a gluten network. The important thing is not to mix too long, as you risk to break the network and lose liquids. Also, don't be scared if the dough is sticky at the beginning and refrain from adding flour. It is good to mix for 2-3 minutes then let the dough rest for a minute or two, then restart mixing: this helps develop the gluten network.

The more you hydrate the dough the more careful you'll have to be when working with it, as it will be more sticky and difficult to handle. Look at the first two videos in this page to have an idea of the consistency (sorry it's in Italian, but you should be able to understand what's going on easily). The dough in that case was made with 400g flour and 350g water, so 87.5% hydration.

Of course for all of this, a planetary mixer helps immensely.

Your ratio is correct, though only because 700ml weighs exactly 700gr. Incidentally the water ratio is known as the 'hydration'.

Water ratios tend to range between 50% (a dense loaf) and 80% (ciabatta). A high water ratio is important in getting nice big wholes in the bread (an "open crumb"). The first response when you make a dough with these ratios tends to be "that can't be right" and to add lots of flour. However, bread dough is supposed to be wet and annoyingly sticky (even at 50%). It takes practice to handle dough in that state, but it can be done. Just make sure that you have a good dough scraper.

Here's a video of someone showing one of the many ways to handle a wet dough: http://www.gourmet.com/magazine/video/2008/03/bertinet_sweetdough

I am a mixer in a bread factory in Nigeria. I use 55% water for most flours, though some take upto 57% depending on their protein content. However we use mix & mill/knead machines with chilled water to get a smooth & tacky dough.

After doing some research I found that most flours labeled "bread flour" also contain diastatic malted barley flour. Further, organic whole wheat flours that I have found locally (Pacific Northwest) don't contain the malted barley flour. It turns out that diastatic (uncooked, still containing amylase) malted barley can be purchased on line in small quantities. The bakers' formula amount would be 0.5% or about two teaspoons per 1000 g of whole wheat flour with 70% hydration. Along with 1.2% to 1.5% instant(not instant rapid rise) yeast a dough that is tacky but not sticky is produced and, after baking, a bread with excellent crumb.

I use two different hydration formulas to bake bread. When baking whole grain breads I use 90% hydration because those flours absorb water more so than white flours. I make ciabatta using King Arthur Bread Flour at 80-85%, depending on the humidity of the area. I first mix with a wooden spoon to incorporate all ingredients well, then cover until doubled. Then I use the fold and stretch method to proof, shaping my loaf in a lined and floured basket on the third proof. Rolling out on to a pre-heated baking stone, I slash it 3-4 times with a sharp razor and bake at 450 about 25 minutes. Test the inside temp for at least 195 degrees.

There's a great article written by Gareth Busby. It explains how the protein content of the flour is related to the amount of water required - a 1 gram increase in protein equates to an additional 2.5% of hydration.

You can use the recommendations alongside the Hammelman's recommendations shown above.

I like his stuff because he breaks down complicated points in baking really well.