"many suspension frames have too much chain growth for just 2 extra links or even 4"
Maybe from unsagged, but if you can work out where the effective chainstay length is at its longest and then hold it there then biggest-biggest + inner+outer links length overlap will work?
"on a hardtail chainstay length variance "
"One chainlink is an inner and an outer link"
"big / big plus two links "
I see, Ive been thinking your 2 links are one inner and one outer
So youre setting up with 4 chain pins worth of length, when you could also use 2 chain pins of length (one of your chainlinks, rather than 2)
"one link shorter and it would not go into first without straining the mech"
I do with mine, they all seem alright
"MAY leave you with too short a chain. This is because on a 2x9 or 3x9 etc you should never actually use the biggest chain wheel to biggest sprocket, whereas on a 1x9 you obviously will do"
For 3x9 I wouldnt for crossed chain innefficiencies, but for (bash)2x9 I do go big-big. On short sharp up/down/up down type trails I leave the front mech alone and shift only with the rear
1x9 I go that short too?
How MAY it leave too short a chain?
"so mech length does make a slight difference too"
IF you set it up with excess chain like you describe, but if as short as will go with biggest-biggest + 2pins/rivets length, no?