As title, i'm trying to get the correct chain length and i'm now trying this approach (embarrassed) 🙁
Just need to know how many links you have?

I'm running a blackspire stinger and a medium cage slx rear mech

Big to big plus 2 links works for me. I have never counted them. When it is all set up I double check that small to small works and the mech is able to take up the slack.

But what position should the mech be in when in big to big?

nearly full stretch or like me the lowest 2 gears are unreachable

it doesn't go around the mech when you're sizing it. between 4 and 5 o'clock when fitted round mech worked for me.

Having run this set up on an older model Heckler, i'd advise making sure the chain isn't too short with the suspension fully compressed. Mine was set up so i'd probably break the mech on big/big at full compression. Shifted nicely and rarely dropped the chain. You just need to avoid using 36t front 34t rear.