Right that's exactly what I've done. I had a look at Sheldon browns page and I don't have the bits he used to measure the rim. Is there a standard spoke bed thickness measurement I can use or are the spoke length tolerances too small to risk that?
Is there a standard spoke bed thickness measurement I can use
Not really, no. If you're going to do it properly and measure the ERD then there's no point guessing part of the measurement.
@Brommers started
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Right that's exactly what I've done. I had a look at Sheldon browns page and I don't have the bits he used to measure the rim. Is there a standard spoke bed thickness measurement I can use or are the spoke length tolerances too small to risk that?