You must mean the belly side of the riser.
Anyway, it really doesn't matter which way you measure it, as long as you measure brace height to the same point every time. For example, the brace height of my Turkey Creek is right at 8". But unless I told you that I was measuring it at the back of the bow, you might have some trouble matching that measurement. It's only 6 3/8 to the deepest part of the grip. If I were to email Doug and ask him for the brace height range for my bow, he would tell me something like 7 3/4 - 8 1/2 inches (I don't remember exactly), but he would also mention that he measures to the back of the bow.