Originally Posted by oldbobcat

When you understand that the main determinant of cockpit length (span between saddle and handlebar) is the torso, not your forearm, you can see the fallacy of this measurement.

Here's how I do it. First set the saddle setback and height comfortably. The lower your torso, the farther back the saddle needs to go. Second, have a friend observe your profile. Sitting with butt firmly in saddle pocket, back extended comfortably, hands resting on brake hoods, and elbows moderately bent (down, not out), your upper arm (humerus) should make roughly a 90+ degree angle with your back. If the angle's shorter, you need a longer stem; if it's longer, you need a shorter stem.

There's nothing magical about this angle, it's just a relationship that seems to work, so it's a starting point. You're trying to find a hand position that works for steering the bike, balancing the upper body, and generating power for sprinting and climbing positions, and there is a bit of compromising necessary.

Interestingly, this fitting technique puts my bar about 5 or 6 cm beyond my fingertips, and my arms are quite long relative to my torso height.