Here's the approach I take to measuring rim ERD for the purposes of spoke length calculation:
- cut two old spokes to exactly 200mm long (cut to 150mm long if you're building BMX, 20" wheels etc.);
- loctite on the end of these spokes the nipples you will be using with the build, ensuring that the spoke is engaged properly with the nipple head;
- insert spokes into rims at opposite spoke holes and measure the gap between the cut spoke ends;
- add 400mm to this gap and you have the ERD of the rim;
- measure the rim in several places to make sure you've measured correctly.
Here's what the spoke / nipple engagement should look like. Once your wheels are built up you want the spokes to fully engage the spoke head like pictured below: