While many wheelbuilders utilize diameter + thickness of the spokes heads, i.e., measure to the top of the nipple heads - remember the key word is "effective". I.e., if the number used works without any functional issues - then that's the particular wheelbuilder's ERD for a given rim.
My ERD's for example all aim for the screwdriver flat of the nipple. And I note that when providing ERDs so folks can decide if mine will work for them as well. Mavic does the same for standard profile nipples.
So it's really better to say: "Effective Rim Diameter. This is usually rim diameter..."
This way you are not implying there is one standard or one way for measuring ERD.
=8-)
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5000+ wheels built since 1984...
Disclaimer:
1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:
Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life