Not very.
Remember that when you are set for long convergence, if you get a close shot your hits will be spread out and less effective. Also, the "hit" point will be a bit above your gunsight pipper, because convergence allows for bullet drop in order to make the bullets converge at the stated distance.
Most important, if you really take most of your shots at 600 range, you are losing some damage potential since the rounds will slow down and scatter at that range no matter what your convergence settings are.
I put all guns to converge at same spot, usually 300. If you notice, many responders are tying their flying style with the convergence. Boozer likes the BnZ energy style, and his convergence is long to be deadly at those ranges. Other guys have it in close, probably because they fly turns and get lots of in close defelction shots.
Remember that convergence is useless if you dont hit the target. My advice is simple: watch what the range is when you get good hits, and set the convergence toi that range. That way your hits are focused and deadly!