Measure it along the length of the body ... the steeper the angle, the shorter the overall length for a given stem tube length. (i.e a 120 mm 17 degree stem might measure, center of bars to nut, 100 mm and so would a 0-degree 100 mm stem ... but the angled stem would have a longer stem tube. I do believe that is what Fietsbob meant.)

(Rationally, I knew the Great Stem-length Misdirection Conspiracy would be uncovered eventually, but ...

No one expects the Stem-lengthish Inquisition!)