The key is learning that you have the ability to control how close an enemy flies through your guns. Once you learn how to create and exploit these incredibly close crossing angles, you will start to see the applicability of the 30mms. Whenever you try to set up a nice crossing shot and it turns out to only be 300yds on the cross, ask yourself if you could have gotten that bogey in closer and how could you do that. Getting your nose out as far as you can is the general rule of thumb for bringing him in close. I like to use the flag pole concept as a training point. Imagine a very long flag pole extending out of your enemies nose, say 2-3 times the length of the aircraft. If you track that point in space at all times, he will draw in very close and you will be lined up with him also.
I recall two instances of unwittingly doing such a thing in a K-4. In both cases, I heard the explosion and saw the kill award without being able to see the actual kill. Doing that systematically has got to be tough. OTOH, I roped a Mossie in one not too long ago. It was relatively easy to close on him after the reversal and blow his wing off. He'd already given up any chance, though, and, honestly, what kind of Mossie pilot follows a K-4 on a steep climb?
Generally, with the K-4, I fire and wonder where the hell the shell went. Not so with 3x 20's banging away - or even 1. But, I'll try your 2-3 length thing. I still enjoy flying the k-4 and it's a damn good escape pod, should I screw things up (highly likely). It's also a fine bomber killer and doesn't run out of puff like the G-14 does up there.