Folks can do it any way they want with their own bows, but the 'standard' is to measure from the string to the deepest part of the grip, then add 1 3/4". This is because your draw length doesn't change, even though the depths of various bow handles do.
As far as on the tree with an unfinished riser block, I would draw the handle profile on the riser block, measure 1 3/4" up from the deepest part and mark the wall. Then measure from there, down, the length of my draw length. And that's where I'd draw the string to for final tiller and draw weight.
While drawing bows on the tiller tree, I draw the string from where my middle finger will be when I draw the bow by hand.