What I do is look at three things on the new frame and compare to a bike I have that fits: "horizontal" or "effective" top tube length, and seat tube angle. Top tube needs to be looked at in combination with STA . I also look at head tube length because I don't like a lot of saddle-bar drop and there's only so much height you can get from a threadless headset system. People who like a lot of bar drop would look at it in the opposite way. But again, I know what the geometry of the bikes I have that fit is and I can look at a new frame and consider if I can make it work with what's available for seat post set back, stem angle and length, etc. Head tube angle, fork rake, etc. can impact handling, but not fit very much.