3×9 set up – long cage mech.
Fitting new chain tomorrow but think old chain might be slightly too short.
How many links should it have ideally?

Big cog, big ring, miss the mechs. Add one pair.

See Sheldon. If it’s a FS, let the air out the shock & compress it first.

Just had a look on Youtube – saw 3 different ways of doing it.
Onzadog – your way was one of them.
Some Aussie dude was doing little cassette ring to little chainring through the rear mech – surely that can’t be right?!
Who’d have thought there could be so many opinions on the best way to fit a bike chain 😯

Long enough that the mech doesn’t rip off through the travel when in Big to Big.

I gave up with the other methods – that seems to work for me.

Big-to-big missing out the rear mech, then add two links. Don’t forget to include any powerlink/missing link etc in the calculation.

When complete and chain fitted through the mech, check that you can select big-big without seizing up the drivechain 🙂 (do this on a bikestand not riding along)

http://www.parktool.com/blog/repair-help/chain-length-sizing

Onzadogs method is the way to go, always used it and never had a problem, and as already said if its a full suss compress the shock whilst doing the measuring.

Dumbass question, but does the compressing the rear shock thing matter if it’s a single pivot (Meta) as the wheel never travels backwards in an arc like some full sus designs?

+1 big to big +1
All 8 bikes in the house set up like this,

In theory, yes, you need to cycle the suspension to check the longest point. Possibly most important on a high single pivot bike as that would suffer the most chain growth. In practice, it doesn’t make too much difference.

Besides, you’d have to be in big big and bottom the suspension all at the same time for it to go wrong.

Done.
All the old worn stuff binned – new shiny chain, cassette and middle ring added and shifting lovely.
Shimano really do do things well don’t they… 🙂