Yeah... having fit a lot of people for production bikes... the numbers stamped on the frame are guidelines for figuring out which bike to start their test rides on.

They don't refer to much on the bike anymore... most companies (but obviously not all) will call a bike a 54 if it would fit the same person that a square geometry 54 would. They could do it by top tube, sure... but that only tells you part of it, even for the most basic fit... what about the head tube height? More and more I'm coming to discover that that's a variable that I need to pay more attention to. In the end, the V-top tube length doesn't tell you that much more than the faked tube length.

Like massive is saying, if you want a number that tells you if the bike fits when it comes to production bikes, it doesn't exist. You have to test ride the bike, or sit down with a full set of dimensions and compare to what you know works for you.