Best thing to do is to remove the shock and lift the swing arm untill the chain is at its tightest (can also load a few fatties on the bike to do this as well) and tighten up the chain in this position (not supper tight, just a bit of movement) then put the shock back on and put the bike on a stand with the wheel off the ground, select your spot for measuring (i use the swing arm at the end of the slider) and measure what the slack is, and now you know where to measure and what the slack must be - you can even make a little plastic template to use for checking