To muddy the waters even more - I'm very much a long leg shorter torso rider - and a 56 P2C fits me very well. One thing I really liked about it was it had a long seat post, so I could get high and steep on it. It does all come down to fit. I would say that rules of thumb are good to narrow the field, but in the end fit is a very personal thing. Best practice is to try all of the frames you are considering and then go from there.