Rule #1 The 30mm has a mind of its own

But you can help it out.
The key is setting up your shots. Its best for crossing and deflection shots, or shooting bombers. If you are chasing 6s, forget about it. It works best to get planes nice n slow in the rope, then flip over on them and shoot em as they are floating. Or another good way is to get them nice n slow, force an overshoot, and shoot them as they cross in front of you. Always shoot about 1 second earlier than you think you should with a nice lead. The bullet drops quick. I keep it at 400 to avoid too much quick droppage. The lower your convergence, the further you have to lead, the higher the convergence, the more it lobs. Its beneficial to come up under the plane you are chasing and then shoot as tour nose crosses above their plane. That way the bullet lobs into them.
The key to flying a 109 is very similar to how you fly a Ki84. Fly it the same way using vertical loops and quick in and out shots. This helps you get better ropes and better shot opportunities. If you are trailing a guys 6 for longer that 5-10 seconds in a heated battle than you should pull off immediately and re-adjust the fight to force crossing/deflection shots rather than direct 6 and bank lead shots. The longer you chase the more you waste bullets and the quicker you get picked. You can catch them, get in close and lead on the bank shot, but if you overshoot, pull up into vertical immelman immediately and come back around but make sure to check your 6.