Instead of the direct measurement, what counts on a recurve is the length when following the curves of the limbs. Take a cloth tape measure and measure from nock to nock along the back of the bow (side away from you when shooting). When you get that measurement, subtract 4" and that will be the length string you need. Most custom string makers build to actual string length instead of to the old AMO standard.

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edit: Chad, ya beat me to the draw this time and got in a post before I did.