Originally Posted by
Andrew R Stewart
Spoke elbow to rims ID then add up to 3mm. About 2 for the nipple head and 1 for the crossing's added distance. Andy
This will come op short because you forgot to allow for the rim's wall thickness. Unfortunately, this can certainly a sizeable variable, ranging from 2 to 6mm depending on the rim itself and whether it has eyelets.
If you measure to the end of the nipple and add the nipple length, as mentioned earlier, you'll be spot on. Of course nipple come in a few lengths, but you can usually know which BTW sight.
On most rims, I use my X-ray vision to locate the top of the nipple, and measure to there against the side of the rim.