My experience is that lower BH means more power stroke, less energy left in the bow. The bow shoots faster, smoother, and quieter the lower I go. But at about 5-7/8 or so, I start smacking my armguard at the wrist. In hunting temperature clothing, it starts smacking farther out. The bow is quiet until the string smacks the armguard. A fistmele for me is right about 6-1/8. So I find that right about 6" works perfect for me. Maybe if I found a way to grip the bow a little more from the side I could go lower, but why mess with what seems to work?
Measuring from the back instead of the belly side of the handle makes sense from a bow performance/consistency in measurement among various hand grips. But since my limit seems to be driven by the distance from my wrist to the string, the traditional way of meauring seems to be the way to go.