Wheel build sanity check
My method for measuring ERD is simple for a normal wheel or a deep section aero wheel.

Feed a white or light-coloured string through two spoke holes directly opposite each other.  Count the empty holes on either side to be sure.

Then thread a nipple onto the string on one side.  Knot string behind nipple and pull string so it sits in the proper place.

On the other end of the string, thread enough nipples so that the first one pokes through the rim and sits normally.  

Use hand tension to take the slack out of the string, but not so much it stretches.

Use a permanent marker to dot the string where it enters the second nipple.

Release it all, and then measure from the knot to your dot, and add the height of one nipple.  

Note if you have access to the base of the second nipple then dot there and don't bother adding an offset of one nipple height.  

Then repeat this in two more places around the rim - bare rims can be quite out of round, so averaging three results should get you a more accurate number.