The armament is fairly weak, at least the C.202 you have an option to add a pair of 30 cal machine guns which still gives it fairly weak armament.
Basically you have to sucker someone into a turn fight, and pour lead - depending your aim if you can go for a wing it would require less ammo then just spraying, and forget about snapshots being effective.
I had one excellent sortie in a c202 and banged out 7 kills, I don't recall ever since doing better then 3-4 on a typical run if I get extremely lucky.
I generally aim for less then 200 - rarely fire above 400 yards or even at 400.
If you can slip behind a spitfire and straddle him, you have a good chance of pulling a few shots off before either climbing out or running off to extend.