The thing with convergence is that what ever you set it at is the range you should shoot at.... just one more thing to think about while your flying your plane, using flaps, adjusting throttle, kicking rudder and so on and so on.
What works for me is to film a bunch of fights. Find a furball and take your best shots. After you have a couple dozen shots taken go watch the film. Pause it at each time you pull the trigger and check the range in the list of the target your shooting at. Do this for everytime you pulled the trigger, the more the better to get a good sampling. Write down all the ranges for each shot. Im willing to bet they are all going to be pretty close to the same range.
You already have a picture in your mind as to when you say to yourself "Yes fire now!". Thats why Im thinking they will all be about the same range. Now as long as that average range is under 450 your good and can set your guns to that convergence. Your already shooting at that range, so nothing to think about or learn, just do what your doing. If that range is OVER 450 will then you have some work to do because most rounds loose a lot of their energy out past 450 as well as figuring the lead with the drop that gravity adds in. I forget who said it but, "when shooting get close, and when you think your close enough, get closer THEN shoot". So while you can hit at 600+ and it will do some damage your basically wasting rounds.