on a hardtail i put the chain from large chainring to large sprocket (without going through rear mech) + 2 links

but what about a fs?

same, IIRC unless you have one of the few FS bike where the chain length extends as the suspension compresses (can't name any myself).

If you've got a coil shock, take the spring off and see how much the chain length changes as you compress the suspension. Most designs have some chain growth, but not many have more than 1 link's worth.

I'd find the point where the chain is at its longest, and then do your usual fitting method.

What paule said, air out of shock/coil off, shock fully compressed, big big plus 2 links to be on the safe side.
Most full suspension bikes have "chain growth", some more than others.
1st generation VPP and single pivot bikes are ones that benefit from measuring the above way if don't want to destroy a chain/rip a rear mech off…

don't nearly all FS bikes have some chain growth in the axle path?

Yep, some more than others. Its usually the longer travel bikes that have the most chain growth.

Most designs have some chain growth, but not many have more than 1 link's worth.

Well I'll bbe a monkeys uncle, I wasn't aware of that, I though t that only a few older designs induced chain growth…

Unless the bike pivots around the B/B or is a gearbox bike, there will always be a change in chain length to some extent.