Detroit, what zak is quoting is called the "wingspan" method of determining drawlength, devised by Bernie Perrelite. I tried it and it left me a bit on the short side so I came up with another version. Measure your wingspan and divide it by 2.5
That left me with a measurement of 67", divided by 2.5 That comes real close to the 27" that I presently shoot which is real comfy for me. I used to be like most guys and shoot too long (28.25"). Made a world of difference in aiming ability and accuracy when I finally got it right.