There are three different places where cast off/cast on is measured to insure you get the right fit. There is cast at the heal, or top of the butt. There is cast at the comb which is measured at the bump or the highest point on top of the forward part of the buttstock. And then there is cast at the face which is the most important one. It may be that in general it is believed to be about one third of the way back from the point of the comb, but a good stockmaker will move that point to corrispond to the individual's face placement on the stock by measuring and observing the owners gun mounting.

Toe In/Out is another important measurement and it tells the stockmaker how to measure for the cant of the toe portion of the stock. This will also vary according to the person's physique and is most important on large busted women, they need more toe out to protect themselves from being gouged every time they shoot. Toe measuring also takes into consideration the general body shape of the shooter including the size and placement of the shoulders. A body builder with large Pecs might need more toe out than a little skinny guy with sunken shoulders.

A good book to read for those who are truely intersted in this subject is "Gunfitting" "The Quest for Perfection for Shotguns and Rifles" By Michael Yardley. It expalins all these things in plain terms better than I can and is well worth the price for gunmakers and stock fitters.
Bob