There are some birds I'll have the convergence pulled in tight, like the Zero and the Spitfire, and other birds I'll have it relaxed a little more, like the Hellcat and P-51.
I've been messing around with my convergences for a long time and I think I've settled on around 425 to 450 for all six .50 cal guns on the Pony and Hellcat. At least, give it a try and see how it works for you. The key is that with the machine gun birds, you've got to hit with your bullet stream concentrated enough over a long enough time to do the fatal damage. If you set your guns out to 650 then when you're shooting at 300 yards the bullets are too spread out and are less effective. With a setting of 425 or 450 they're still concentrated enough at the shorter ranges, and at the common 400ish yards range they're deadly.
ps: my longest convergences are on the 262, where I have two 30mm cannons set at 650 yards, and two 30mm cannons set to 550 yards. The reason for the little difference is that they're still concentrated enough for mostly instant buff kills at the angles I dive on bombers at, and the slight disparity in range gives a little vertical spread that helps when firing at fighters from directly on their six, where the profile is the narrowest and hardest to hit. The long range is to get extra loft on the bullets to help when pulling high-lead shots.
