Modifying AS's method to elimiante guess work about the rim's thickness at the nipple.
Measure any spoke from the inside of the elbow, (or the center of the head, less 1mm) out to the outer edge of the rim. Then remove the tire, lay something across the rim and measure down to the top of any nipple. Subtract to find the length to the top of the nipple, and subtract 1mm because you want spokes ending short of there.
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