To get the AMO bow length, you measure like BC said--start at the top of the nock groove, follow the contour of the limb (but measure through the center of the riser), to the top of the other nock groove. You can measure it strung or unstrung--should get the same number either way.

AMO says 3" shorter for the string, but it can vary depending on the bow and your preferances.

What kind of bow, and what is your brace height with the new string?

Chad