I am following up on this thread. I have found a few rims that match the ERD (604) of my rim, and if necessary I can use one of those. But I would like to consider others as well. How much leeway do I have in the ERD for my rim? If I get one with an ERD of 604±X, what's an acceptable X? Or maybe it's 604+X and 604-Y where X!=Y. Or maybe I'm overthinking it. :-) So, in summary... how much leeway do I have?
Unless you measure your spokes and calculate for the hub and ERD, you have no certainty on leeway.
It depends entirely on the spoke length. One build might have spokes just long enough, then a new rim with a 1mm bigger ERD would need new spokes. A different build might have spokes that are just off being too long, then a new rim would need an ERD no smaller than the existing rim.
I wouldn't want to change more than 2mm in ERD, ideally either exact or 1mm, personally though, I would want exact or new spokes. This is slightly biased given that I like building my own wheels.
If you check the spoke calculation for a rim of 604 ERD and a hub and you change the ERD by 4mm, using the same hub, the spokes change by 2mm (aprox). This was on a 32h, 3x front wheel.
2mm is enough to make the wheel snap spoked more than it should is too small or not be able to tighted properly if too long.
Now, whether X=Y or not also depends on whether they rounded up or down on the spokes. ie 290.7 being rounded to 290 or 291. Some people round down, osme round up and some use the maths approch to rounding.