I think TinyTim has mentioned one important thing - hit ratio is just one aspect of air combat. If you fly in a way that allows you to achieve exceptionally high ratios, you'll have to sacrifice other aspects - like quickness of kills or your own safety. It's easy to achieve a 40% hit ratio against a bomber, when going close and from an easy angle. But if your focus is on not getting hit by the bomber gunners, you'll have a very hard time getting a hit ratio that high.
Personally I can achieve 10-15% without focussing too much on hit ratio, depending on plane and target.
Best ever was 100% with a Yak-9T killing some 109's. The NS 37 can be used as a single shot weapon to great effect.
