I asked this question not long ago and got an inexplicably hostile reception in the forum. Being of an R&D background I never understood this attitude. If we all did only what was tried and true we'd all still be riding horses, not bikes. Big tech companies fit their products this way, ie, airplanes, cars, etc. The problem is the number of man hours required would make it cost prohibitive to do it for a single person unless there was a lot of money involved such as racing the Tour de France.

A mechanical engineer acquaintance explained that bike frames need to be analyzed using finite element analysis and that it is not trivial. There is software out there to do it, but there's a good reason why custom bikes are expensive: even experienced craftsmen who can eyeball the general shape and materials of a frame sometimes get unhappy results like flex, etc. Unless you have megabucks like Trek, Specialized, etc. there's a lot more art than science in this craft to make a bike that not only fits right but "feels" right to the user.

http://en.wikipedia.org/wiki/Structural_analysis