Originally Posted by cadillacmike68

Instead of cutting the spokes so you have a large gap andf three items all to keep straight, use two uncut spokes (of say 300mm but measure them), thread the nipples so that the top of the spoke is flush with the top of the nipple put them in oppoisite holes and measure the small gap you'll have. add that to the length of the two spokes and you have your ERD.

If the clincher is a same style rim (i.e. both are box style non-aerao, etc.) then the ERD should be very close. up to 2mm should be ok, larger than that difference and either they'll be too short na and might fail or be too long and won't tension up enough.