Simplest way is; While the bow is unstrung, take a piece of string, hold it at the top of the nock grove, the run it down the back of the bow. Making the string follow the contours of the limbs. When you come to your riser, dont follow the contour, run it straight across, then follow the contours of the other limb. When you get to the top of the next nock, cut it or mark it. Then measure your string. Subtract about 3" for longbows and 4" for recurves, and you got your AMO. I simply use a flexible tape measure and just use it as my string. I clamp the 0" mark at the top of nock groove at the limb tip with a spring clamp, then follow the contours of the limbs.