honestly, I think measuring from the belly is more ideal. However, I understand the standard is usually from the back of the bow. I figure if I measure from the belly than at least there is an automatic safety built into it. I want to speak about draw lengths. I've built 5 bows that would have all been good functional shooters at 28" draws. But I wanted a good safety margin and tried to tiller them out to 30". All of them broke around 30". SO if you are going to account for a safety margin, which I think you should, design the bow for that margin. Don't design the bow with 28" and thing that you can get two more inches out of it.