I'd measure from the hub (elbow) to the nipple. Add nipple length (usually 12, or 14 mm). Then deduct the length between the top of the nipple, to the end of the spoke on the inside of the nipple. If a spoke protrudes through the nipple to the other side, or is 3, or more mm beneath the tyre end of the nipple, the original spokes were too long (or too short in case it ends too "deep").