Cast-off is usually measured at the butt. Yes, the comb is moved over, so the face better lines up with the sights, by moving the centreline of the butt to the right for cast-off and to the left for cast-on, as I understand the term.

How much is required, depends on the individual the stock is initially made for as well as the thickness of the comb, weight of facial bones, fat, etc - not by any 'given' set of hard and fast rules.