1) screw a nipple onto a spoke backwards.
2) shove it into a spoke hole, and mark on the spoke the point where it's even with the top of the rim edge (bead seat.)
3) Pull it out, measure the distance from mark to shoulder of nipple.
4) Measure the outer diameter of the rim.
5) deduct 2x 'nipple depth' measure from total diameter. Done.
I started with 2 310mm spokes. I cut off the elbows and locktited the nipples so they were barely threaded on. They were long enough to overlap when inserted into opposite holes in the rim. I took a known rim and figured out what to subtract the overlap from and done. I used to use the Wheelsmith ones, but I like the spokes better. They use actual nipples which consistently seat in the rim the same way and the smaller diameter of the rods are easier to read on my pocket rule. I get more repeatable results with them.
If you can't find the 310mm spokes, you can get 2.0mm wire from a hobby store and use some sort of super glue to get the nipples on one end. Calibrating them is the important part. Pick a well known rim like an Open Pro that is also consistent. Velocity rims are all over the place with the ERD. You might get it wrong once, but once they are dialed in, you always have a very accurate method. I measure each and every rim I build. Nothing is more frustrating than getting the spoke lengths wrong.
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