I think of this as being about locating my shoulders properly in relation to the seat and doing so with comfortable arm bend. So, if my shoulders are supposed to be in one certain location and my arms swing an arc about the pivot of my shoulders, that means that any location of the handlebars that puts my hands on that arc will not change that location. Now, for the short portion of that arc that is practical, the arc is nearly a straight line. Very conveniently, this line has a "slope" of 2 horizontal to 1 along the steerer. So I can shorten the stem two cm and push it down one for the same shoulder position (and a rather different, but also quite suitable ride). Now I vary that line's height for different bikes with different purposes.

I have that line (and many bikes I have and have owned) drawn on a CAD program. I use it to set up bikes and see if prospective bikes will be good for me and whar stem I will need.

I know this sounds extreme but as one who has arms long enough that most bikes out there require custom stems or a "B" fit or worse, that drawing has been a real blessing.

Ben