To my knowledge, there aren't reverse spoke calculators. but this might help. Spoke length changes by 1/2 the change difference in ERD. The hub calculation is different, and more complex, and depends on the amount of cross, so take a shot at what looks like a potential rim, then try a few flange diameters, and get a sense of how spoke length changes relative to the hub diameters for any cross.
Don't worry about flange to center measures, they change only about 1mm for every 10mm. When you get rim and hub you think will work, you'll recalculate the spoke length with each dimension accurately entered as a double check.
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