When I first tried to build a wheelset, I didn't have any spokes to use to measure ERD, so I had to hunt around on the manufacturer's website to find out what they said (Mavic likes to hide this info, especially about old rims), but it turns out what they said worked for me. Now that you've got some spare spokes, keep them so you can measure ERD for yourself in the future.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"