[Cobra]

To answer your question, yes this is too simple of an approach. The problem is you are counting many of the hands twice. For example if you count flushes and then add straights to it you have counted twice the hands that are a straight and a flush. However even with these double counts your approximation of a hit seems to be way to high, what do you define as a hit did you include any pair. If you did there are many double counts with a pair. You can get a pair with four to a flush, a pair with eight outs to a straight, etc.

Cobra