The 2 biggest factors in figuring out the best convergence seem to be 1) personal preference and 2) what guns you're using.
In the past when I flew strictly American planes with the 6 .50's I had my convergence set anywhere from 400-450 or so and could get good results at those ranges. This was due to the excellent muzzle velocity and flat trajectory of the Browning .50 cals.
When I started mixing up my plaen choice and flying German crates I had trouble hitting anything at those ranges with the slower firing 20mm cannon and brought my convergence in on those to 300 I believe it was.
Now I'm back flying U.S. but I've brought the convergence in to 300 on those planes as well and it's amazing what you can do to a enemy aircraft with 6 .50's at that range and closer you can just shred things.
If you have the patience, set your convergence at the max effective range of whatever weapons you are using (I'd say 450 for the .50's and 350-400 for cannon) and then try it for awhile and then move it in slowly as you get better. Ultimately it's all about experimentation and seeing what settings give you the best result for your style of fighting. Good luck.

P.S. when flying planes with multiple sets of guns like the p51, f6f, p47, etc. most people here I believe (and this includes myself) set all the guns to converge at a single point rather than stagger them at decreasing intervals like 350-325-300, etc. Hope this helps.
[ 10-27-2001: Message edited by: Soulyss ]