“Unless you are a professional rider you probably wont even notice a 3 psi difference. I don’t think it is even possible to be that accurate with the psi in shocks with your average shock pump. Would aim for around a + or – 10 psi accuracy at best.”
I wouldn’t expect to be able to get brilliant readings with some shock pumps but I can certainly feel quite small differences. Last week I was tweaking my suspension after a service and after getting into the right window of about 5psi on the fork and 10psi on the shock the finest adjustments were literally just connecting and disconnecting the pump to let out a tiny amount of air.
I wouldn’t set suspension based on expecting to reach full travel frequently, there’s a lot more to it than that – how the bike feels and sits at sag, how it feels over rough stuff, what happens as you pump and preload and hop, what happens when you land bigger stuff, etc etc. Full travel on the shock is fairly common for me but not on the fork, but it’s usually from flatter landings on drops, not jumps with transitions (although much as I like getting airborne frequently I don’t have the confidence to go high/far).