Light bicycle has on other occasions listed nipple seat diameter instead of erd so you are probably doing it right making your own measurements..
http://forums.mtbr.com/wheels-tires/new-light-bicycle-38mm-rims-941499-2.html
What does "ERD is Mavic's Nipple Seat Dia + 3mm for nipples" mean?
Thanks Svenhöek, that's very helpful and reassuring. Will be ordering CX-Rays so could have been an expensive mistake!
Will double check in the morning to see if it is 4 or 5 mm out.
@Goyt started
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Light bicycle has on other occasions listed nipple seat diameter instead of erd so you are probably doing it right making your own measurements..
http://forums.mtbr.com/wheels-tires/new-light-bicycle-38mm-rims-941499-2.html
What does "ERD is Mavic's Nipple Seat Dia + 3mm for nipples" mean?