Hi all:

I've always used the "big-big+1 full link" method to size my chain on double drivetrains. I've never setup a 1x before, and am finding different suggestions online. Some places say the same thing, i.e. big-big+1 full link, where others say you should add 2 full links.

Obviously, it's always easier to shorten than to lengthen, so I'll try the +2 full links first, but was curious what others are doing, and what are the benefits if any of +2 vs. +1.

Thanks!

If you have a "strategy", it'd be dependent on the chain wrap capacity of the RDER and difference between smallest & largest cogs.
A longer chain will spread the wear out over more links and since you don't have to worry about the difference up front, you can add quite a few extra links.

The RD is an 11 speed XT, specs say 33T chain wrap. I have an 11-40 cassette and a 36T chainring. I just tried it with 2 full links and it's working just fine, but there is plenty of chain to spare. I'll leave it like that and see if I get chain drops. Having a bit of slack will allow me to swap for a larger chainring if I find 36 is too small.

2 and 3x drivetrains seldom use large rear sprockets from the largest ring - so no need for an extra long chain.
They also have a great difference in total used sprocket teeth count when switching from largest to the smallest front chainring.

With 1x drivetrains, there is, obviously, only one front chainring. And it is used for all the rear sprockets.

Hence, it doesn't hurt to have a bit more chain (2 extra links) to not keep the derailleur cage not as highly extended when riding on the largest rear sprocket.
Even though cassettes used with 1x setups are often very large in teeth count range from the smallest to the largest sprocket, there still is only one front chainring, so RD chain wrap capacity should not be a problem (do check though).

With 2 and 3x chainrings, this combo is almost never used - one should shift to a smaller front chainring when using the largest rear sprocket.

I'd probably be tempted to size the chain long enough so that there's just enough derailleur cage hang/clearance in the smallest sprocket (so, bordering on too long). You should have way more than enough chain for the large sprocket (probably more than you'd choose to have), but I don't think more chain than necessary has many drawbacks, at least assuming it's not so long that it drags on the rear derailleur cage. But we've already checked that, so that should be good. Your XT rear derailleur should provide plenty of chain tension. In theory, a longer chain will have more wear life and I think the drivetrain seems to run smoother with lower tension. You can always shorten the chain later (or install a chain device) if you have problems with chain drops.

The new 11 speed XT derailleurs also have a clutch mechanism that keeps tension for bumps and rough terrain. If you drop a chain, make sure the clutch is engaged. Boom! No more problems.

For chain length, I always select the smallest cog & smallest chain ring and set the chain length so that the addition of 1 more link would cause drag against the cage. Then I shift to the second largest cog & adjust the b-screw until there is clearance to guide the chain on to the biggest cog, not against it until it shifts as would be if the b-screw were mal-adjusted. Set right, there will be no detectable hesitation in the shifter.