*** Warning *** this is just "my" take, and YMMV !
My shock is set at 350mm, 450 lb spring, my weight 170lbs, and I think I still need more height.
Shock Length will effect the swing arm angle. If you do not have enough angle when you get on the gas the chain can pull the swing arm up, and cause squat. Where as with more angle you can get an Anti squat effect from shock length. Now Greg is right that spring rate does also play into squat. So you need a spring that will give you about 25-32mm of sag with about 13-18mm of preload. Then you will have to find a ride height that walks a thin line between anti squat, good transition from side to side, and high speed stability. If you bike won't finish a turn with out running wide you need more ride height/spring. If the front pushes entering turns, or it wants to tank slap on the gas you need less ride height in the rear or more in the front.
So 1st get the right spring, then you can adjust from there. With the wrong spring you will never be happy.
What? sorry, I was ignoring you.