I know lots of guys like long convergences. Trouble is, when your convergence is set that long, you lose killing power up close because your rounds will be spread out. Instead of a short burst shredding the enemy, you'll see long bursts with hits spread all over the target.
Also, in planes that have mixed guns and cannon, with convergence that far out the different ballistics mean that at closer ranges EACH GUN TYPE WILL HAVE A DIFFERENT VERTICAL TRAJECTORY. In other words, the rounds with better velocity will be significantly lower that the ones that are slower, so you CANT HIT WITH BOTH AT THE SAME TIME.
As Bat showed, you can get long range hits with close convergence with practice. But, with long range convergence you lose a tremendous advantage in close.
IMHO you are much, much better off setting it in close, learning to shoot at a maneuvering target, and learning to maneuver yourself for the best possible shot.'
And its SO much more satisfying to make them vanish in a blast of bullets, to find themselves suddenly towered.