If you're trying to re-use the spokes (which is not necessarily wise, but say you are), you don't care about the rim ERD. You care about the spoke lengths you're working with.
The spoke lengths in your existing wheel are not necessarily perfect.
The spoke lengths in the wheel you're building don't necessarily need to be perfect, just pretty close. (If you're using aluminum nipples they should be perfect though).
Remove one spoke per side and measure it. From there you can work with a spoke calculator to see what rims you can use if you didn't want to replace the spokes.
Once you have a spoke out, you also know the ERD of the rim you have because you can measure it, then measure the hub and the thread engagement of another spoke of the same set, then use those numbers in a spoke length calculator to infer the ERD.
You can use different length nipples to augment the effective ERD a little as needed.