I get the trotter film to display a tater every other time I play it. I haven't checked (cause from memory there definitely wasnt a tater there) but I'm pretty sure it happens when you've changed the play speed.
There's probably some very academic and formal methods to aim the 30mm.. But let me be honest and tell you what I think is the best way. Stop thinking about it. If you have any habit of hesitating, get rid of it. Even if you have to "forget" or "unlearn" what you know or have taken for granted. The brain never really forgets and in my experience "relearns" very fast. From then, just fire anytime it feels right. Don't think or second guess yourself, just fire and move on. You'll make mistakes and go home bingo ammo early, but just go with it and don't try to analyze it. Just take it all as it happens. Let it become second nature. Maybe.. Forget about the 20mm. Fire just the 30, or concentrate only on it. The best way to learn a tool is to treat it as an extension of your body.
The 30mm is a really dodgy gun and will frustrate you (bullets flying around target despite piper held steady RIGHT on target).. It's a roll of the dice - there's nothing you can do, as far as gunnery goes, about that. Only fly yourself into a solution on as big a target as you can.