For draw length I prefer to look at rear elbow position.
I agree. I've found great variance in what draw length I need, depending on my loop and my release aid. I've mad great efforts getting a release that allows me to use a longer draw length and yet keep my elbow in the same position. Things like adjustable barrel lengths, short jaws, proper fitting wrist straps,forward triggers and the shortest possible loops can allow longer draw lengths with the elbow remaining in the same position.