I don't know, in school (Trinidad St) and in the trade since, we were taught and have used, measure it from the butt. I think if you're having a stock by Holland or such, it's good to know, but the way I see it's like spelling color or prononcing aluminum, they got their way we got ours.
Plus personally I think the end of the butt (heel) can be a more precise point to measure from rather than about a third of the way down. Just dosen't seem quite proper old boy. (should probably put a smiley face or such there)