How much leeway do I have on ERD? 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.