The rule of thumb that I was told many time before is to take your bow length( tip to tip) and nock off 3 inches and there is your AMO length. I first found this out when trying to find a string for my old Ben Pearson long bow. (It measured at 55" if I remember correctly) When I went to my local archery store I was told take off 3" and you will get your AMO, And my old Ben can still knock a flea off a dogs a$$