Well the other thread is locked but my thought on Brophy instep measure is it’s kind of a trivial solution I guess, it basically works out to “mark the bend of the throat (the short heel) as half the short heel measure drawn 29 degrees from the heel” If you take the short heel from the last (and you do it at about 29 deg from the ground) it should land on the last. Where if you take it from the foot and the last has curved away from where the ankle would have been (true for most lasts I think) then that point should float above the last copy/forme. Anyway I think what I’ve heard is for Chelsea type you need more room there than just the short heel anyway or it tends to pinch at the front of the ankle. And for slip ons the needs of the foot passing the shaft dominate. I made my wife do the math don’t go thinking I know how to calculate stuff
