I've built a couple CR-18's in both 622 & 630. In both cases, subtracting 10mm from ISO worked to get the ERD.
However, I use spocalc and round up a bit to get my desired result.
You didn't specify how many holes!
ASSuming 36 holes, I'd use-
281mm or 2nd choice, 280mm
ASSuming 32 spoke-
284mm or 283 2nd choice (very close 2nd choice)
That's ASSuming a 2.4mm hole diameter in the hub.
With a double wall rim, an extra mm long won't hurt, since you don't have to worry about a spoke protruding past the nipple head and poking a tube.