Leow, there is no all inclusive formula to establish a correct draw length. The best reference is to have the bow of your choice set up with rest and nock set. Draw a full length arrow and mark the end taking into consideration of where the broadhead is in relation to your bow hand. Riser design, reflex or deflex, the rest location on the riser and limb design will all reflect on draw length. A very close reference for the fingers shooter without a bow is to get into position as if you are at full draw and anchored. Have someone measure from the crotch in you string hand to the crotch of your bow hand. You will be within ½” of you correct draw length for a fingers shooter.