GMM, I'm willing to re0run my numbers, but please let's agree on an assumption first. What value would you like to use for the distance from the top of the head tube to the center of the intersection between the actual top tube and the head tube? I assumed 30 mm, but it's only an assumption. Let's at least use the same number, since you don't have the frame and can't measure it directly.

Really, another very reasonable question is, how much accuracy do you need? Is the potential use of a 10.5 cm stem versus 10 cm really a big issue? Can't the difference be made up or at least mitigated by selecting a handlebar with less reach?