Not sure if you've read any wheel building books in your preparation for this project, but the one I bought included a very simple and effective way of accurately measuring the ERD of any rim. The author does this by essentially creating an ERD measuring tool out of two pairs of spokes/nipples. I wasn't able to find his published method anywhere on the internet so I don't feel comfortable walking you through it here. (*Edit* See Post #3). But if you're interested, you can check it out, OR just take your wheel to the LBS and have them measure all four rims for you. Remember to measure from multiple places on the rim and take the average length as your measurement.
The book is the professional guide to wheelbuilding by Roger Musson. About $15 dollars for immediate download.
-Jeremy
Last edited by Tunnelrat81; 10-28-10 at 01:31 PM.