Chain length differences between 1X and multi-chainring setups

Current gen Shimano mountain rear derailleurs (dealers' manual) require at least that much chain by their own instructions: 4 to 5 links plus quick link for hard tails and 5 to six links plus quick link for full suspension bikes. Note that Shimano refers to 1/2" section as a link, as is quite common.

I recently set up a chain on a 1x full suspension with RD-M5100 with the 'standard' method, following the instructions from the Shimano MTB chain manual and the chain was much too short putting the derailleur in a peculiar position in all gears and not enough to wrap around the larger cogs, it was unworkable. Following the instructions in the derailleur manual instead, with 5 links plus quick link gave the expected result.

Regardless of the derailleur or drivetrain type in question, the purpose of a chain sizing procedure based on large/large plus one link is to find the chain size where the rear derailleur is so extended that if it were one link shorter, the frame, hanger, and/or derailleur would be destroyed.

Using that as your target chain length has different amounts of merit on 2x and 3x depending on the specifics or your application and the parts used. There is little reason to use that length with a 1x-specific RD if you have the option of making it some amount longer, which you do in most cases.

If ridden in the large/large combo, bikes tend to shift badly and operate with increased resistance from the rear derailleur if the chain is sized this way. The advantages are minimizing chain slap/derailment and saving weight, and marginally better shifting throughout the more useful part of the range (on a 2x/3x) than if the chain was a link or two longer.

Bikes with 1x specific RDs need their large cog combo to be a fully functional gear. And, extant 1x RDs all or mostly all have clutches, so chain length anywhere within the possible lengths that could work will have next to no effect on slap. Chain sizing instructions for 1x-specific RDs tend to target the length to be around the point where in the lowest gear, the center of the tension pulley is modestly ahead of directly below the guide pulley. There is some variance in that, but it will be some positioning of the lower cage that the manufacturer has chosen to perform as intended.

Bikes with non-1x RDs that are being set up 1x start to ask some questions of the mechanic, and the answer will be different depending on the parts and application in question. That could be either someone setting up a parts-minimal 1x commuter or it could be a cyclocross or DH/freeride bike from before clutches and the mainstreaming of 1x. Generally speaking, aside from slap/derailment concerns, that kind of drivetrain is set up to have the longest chain possible that still allows the chain to be tensioned in small/small, but slap/derailment can and often will cause you to shorten it from there depending on the application, to the point where more rough stuff bikes with that kind of drivetrain would often have the RD extend pretty far forward in large/large, i.e. something pretty close to the large/large +1 method. Chain length sometimes requires tuning on that sort of bike in real life regardless of what any manufacturer instructions say.