i think the formula was( might need correction here) 2x draw length plus 10" for stiff handle. and only plus 4" for bendy.
but nowdays i build them to the length that i want and like- as i do prefer a slightly longer bow anyway. as i figure if i can drag my 6'2" frame of pure muscle, sinew and good looks :D thru the bush, i can surely get my 64"to 68" longbow thru with me as well.