'Cast-off is usually measured at the butt.'
That is what I used to think too, but I was wrong. I have been in touch with a man from England, who trained with Holland and Holland; later worked as stockmaker for major English gunmakers, who told me that cast off/on is actually measured about one third of the way between the comb and the heel of butt. Cast off is to the right of center, and cast on (for lefties) is to the left of center, of course. Cast off/on measurement varies, in accordance with variations in shooter's build, face form, etc. but is generally, on average, around 3/8" or less, usually less, at this point, one third distance between comb and heel.