Unless you are gibbonesque - which I seriously doubt (I am fairly - and on the longer draw length side of things) You should be drawing to the edge of the jaw - where the bowl of the thumb meets the hand sort of locks in that spot... I'd not use a half butterfly floating anchor point. You should be able to use fairly standard archery measurements to get your draw length - you'll find through a google search.
Another option if you definitely want to do a floating anchor, or have arms that reach the knees, is to create some sort of over-draw (very popular a few years ago) - which would allow for a massive draw but shorter arrows... basically the arrow rest is set on the inside of the frame by a few inches.
To get the extra speed that may be ideal - the over-draw as you'd have short arrows that would be powered with the main power stroke of the band/tubes....
Edited by mattwalt, Today, 02:18 PM.