Another "formula" (attributed to Hill) was 60" plus the last digit of your draw length. Comes out close to what was recommended above. I'm blessed (or cursed) with a short draw, 25", and after shooting some 40 or so Hills from 60 to 72 inches, I have found that 66 seems really optimum for me. Again, though, I'll sometimes go shorter for more speed (hard for me to get high speeds at my draw) or longer for easier draw on arthritic shoulders. I like real short for take downs, and have had several 60" TD Hills (remember my short draw, though). I build my own Hill styles now and recently did two take downs, one at 61" and one at 62".