i usually get around 31k a game, but i'm slowly increasing and now i'm trying to push it to 35k to increase my credit earnings. you just REALLY need to pay attention and be quick. also, try not to waste gas by speeding up or slowing down, only do it if you need to but even then, it's a waste. remembers, you need gas much more than you need film but also don't pass up the opportunity to get more film when it comes to it.
I don't know how to help with getting perfect shots, I just snap pictures as fast as I can and I usually can manage a chain of Perfects without even really putting that much effort into my shots. I guess from there you just need to develop your skill. But, like, it's not that hard to go all three rounds and it's certainly not hard to get high points throughout the game, you just really need to pay attention to what's going on and be quick to snap a shot and dodge a rock.