Another way we use to do at the shop(since we were primarily a mathews dealer and couldn't change the draw) was to take the average of three different methods. One was the wingspan(which you've convered), second was the yardstick(take a yard stick to the middle of your chestbone and place hands together towards the end, where your middle fingers touch is your draw length), and third was by the recurve with a realease on using a measured arrow. This would normally bring it to within a half to one inch in any direction, then you fit the bow to the average.