1) get close, but not too close. I find that a lot of people will miss if they're at super close range, regardless of their convergence settings. If I had to guess, since he's fairly close to filling your forward view, and you're probably watching closure like a hawk, its harder to judge his movement, and he's not putting proper attention towards aim (figures I'm at D100, how can I miss?).
For me, D200-300 is ideal for a tracking shot (right around D200 for the 30mm)
2) Don't use excessive closure rates. You'll have less time to get into position, you'll be paying more attention to not hitting him than to shooting him, and it can affect the drop of your rounds (the speed of the round = muzzle velocity + aircraft velocity), although the last is minimal in most cases.
3) unload your aircraft. High G, high-speed deflection shooting is hard. I've seen even some of "the" vets miss those shots. I find med-high-six quartering shots (such as will be replicated by a lead turn) to be the easiest, since it offers a nice combination of larger target profile offered by deflection shooting, and easier aiming offered by tracking shots.