You don't absolutely need calipers - a ruler with metric scale is sufficient. Measure from spoke hole to flat side of hub locknut. Then add 1/2 of whatever size the locknut is (what size wrench fits it). Finally double the result. Ex: spoke hole to locknut 30mm, locknut 17mm. (30+8.5)*2 = 77mm. Awkward but effective and accurate. Alternatively you can use a compass or any tool that can accurately span the distance between the opposite spoke holes.