Originally Posted by FBinNY

It's easy to calculate how various stem angles will move the bar.

Start with the head tube angle and subtract it from 90° to get the rise angle a 0° stem, ie. 17° for a 73° head tube. Add or subtract the stem angle to find the stem's rise angle from the ground. Now go online and ask "sine for xx degree angle" and you'll get a number between 0 and 1. Multiply that by stem length to get the vertical rise for that stem.

Do this for both potential stems and compare to get the difference, and add/subtract spacers below to compensate.

Note, that as the stem rises at various angles, it also moves the bar back or forward slightly. However for stems that don't rise at steep angles that change is small enough to be ignored.

There are also various stem calculators, which allow you to enter heat and stem angle and stem length and do the work for you.

As I said, this isn't rocket science, and anybody can learn it.

In any case, at the risk of offending the fitters and their loyal clients, the entire science isn't nearly as precise as they may lead you to believe. a few millimeters either will not make or break you, and once you're in the ballpark you can let your body tell you what next.