The local shop uses a special arrow that has ruler measurments on the shaft. He'll have you draw the bow with that arrow and take the measurement. He'll have you do it 6 times with your eyes open and 6 with your eyes closed.
Place and arrow with the inches marked on it at the base of your neck in the little hole under your adam's apple. Stick your arms straight out on the shaft perpendicular to your body. Where you longest fingers touch the shaft is the length of draw you're gonna need. That's the extremely simple way.