If you've already built a wheel using a calculator and still have it, you have all the info you need.

Start by confirming the ERD, by measuring down to the top of the nipple, and subtract double that from the measured rim OD. The spec will either match that closely or be about 8mm less if it excludes the nipple.

Double check by running the same data set, and (hopefully) you know the spoke you actually used.

Now, assuming you were consistent, note where the spoke ends in the nipple. Look at where the spoke ends and decide if it's just right, or if it's short or long by Xmm.

This is now your correction factor which you'll add or subtract whenever you use the same calculator with comparable data, ie. ERD with or without nipple.


BTW - because I've lost track I went back and read your OP. Your measurement of 533-535 is perfectly consistent with thd rim spec of 527, with 7mm being twice the nipple head engagement. Your 2mm variance is normal because a use inbuilt rims typically a bit out of round.

Lastly, if you're concerned that the thinner spokes will "stretch" more, don't be as the difference is tiny.

So, immunize yourself against the "too much info" disease and build your wheels before you talk yourself out of it.