If "true draw lenght" is a measure to the back of the bow, then it can't differ from AMO by 1.5" to 1.75". The only measure that could vary to that degree is one to the belly, or deepest part of the grip, as one measures brace height. Very little diff in my experience between AMO and the tried and true back of the arrow pass as most stiff "handles" are ~1.5" deep. Exceptions of course for bendy handle bows but they too run at least 5/8" or better so the biggest diff might be 7/8" (ie. 1 3/4" - 5/8"). Don't imagine even 7/8" would make a dramatic diff on an "arc board," if I understand the meaning of that term.