Anecdotally, the formula seems to work pretty damn well for me. My custom is about a 58.75 ST and a 58.5 TT with a 14 stem (which seems a little long now that I'm less concerned about having a flat back and being stretched out). I'm 6'1" with a long torso, long arms (35.5 sleeve), and (I think)shorter-than-average barefoot standover of 33.75" (with a 13 shoe) and the formula spits out a frame size of 59.812885. Coincidentally, I just bought a frame that measures 60cm unconventionally, because it uses the top-of-top tube as "theoretical center" - which means that actual center is almost right at the formula length (top tube has very narrow profile). Gonna have to think about what this is telling me in concrete terms, but I do feel better about the frame size choice I made!