I've got the most perfectly true and reliable road wheels -- which is surprising since they were dirt cheap -- that are perfect for 19-23mm tires. Since I'm well over the clyde' limit and since many of the charity rides and centuries around me include patches of road with more vaguely filled potholes and expansion gaps than surface along with super coarse chip seal, I'm intending to move to 23-25mm wide rims.

I'd love to be able to reuse the hubs and bladed spokes from my existing wheels since 20/24 hole hubs and CX-Rays are pretty pricey. That means ascertaining the ERD of the existing wheels. Since one can calculate the hub dimensions easily on an installed hub, I simply need the spoke lengths. I'm stumped as to how to determine that without removing a spoke; and I do not what to mess up what is now a perfect wheel by trying to remove and reinstall one of the few spokes.

Am I missing a trick? Or, is it futile?