I understood that when I posted earlier. But the ONLY sure way to avoid lacing spokes that are either too long or short is not to build a wheel in the first place. Even with the best information, mistakes happen, and spokes come up off target. It even happens to seasoned builders, the difference is that experienced builders spot the problem early on.

In any case at some point you have to cut or cut bait. You have the best information available in your own measurements and have to rely on it, because nothing is going to change unless you have a trusted friend who can drop by and confirm or correct your work.

So, even though it seems to upset you, I repeat, you can do this if you simply trust yourself and move forward.

Here's one last hint. Spokes generally have 10mm of thread. Look for 12mm nipples with only 7-8mm of thread (sellers don't always know this, but maybe someone here can tell you a brand that conforms to that). With 7-8mm nipple thread, you have 4-5mm latitude in spoke length from 2mm below the top where the 1st thread would show (+1mm if you don't mind a thread showing) to 2-3mm above the top of the nipple before running out of thread. This is only OK with double wall rims.

So, with upto 4-5mm spoke length tolerance, you have a margin for error of 8-10mm in ERD computation, which gives you a decent size target to hit.

So take the old carpenters advice, and measure twice, cut once. You can confirm your own numbers by using 2 different methods.

1- Use 2 spokes threaded to slot and inserted from opposite ends of the rim. Measure from inside the elbow to inside the elbow, and add that to the two spokes to get an ERD.
2- measure overall OD and drop to nipple head, add 1mm to the slot, double and subtract.

Both methods should produce the same value within 1mm (don't forget to use each twice at right angles and average in case the rim is oval). If they do match, consider the odds of getting the same wrong answer by two different methods, and know with confidence that the ERD value is correct.