Formulas don't work really well.
Here's the site I like:
http://spokesetc.com/page.cfm?pageID=35
Doesn't give formulas, but it gives a step-by-step guide to getting htings set up correctly. Within reason, you can always get the seat set up right (unless the frame is waaaay too small and you don't have enough post in the tube). The question then is what kind of a stem you'd have to get to have good posture for the frame you have. If it's too awkward, a new frame could be better.
Getting the right frame depends on the relative lengths of your arms, legs, torso, etc. You defininitely can't use those guides that have you measure your inseam and guess your size if you want an optimal fit. If you've put 3000 miles on your current ride the fit can't be *too* horrible (is it?), and I don't imagine any table or formula will get you closer.