I'd like to replace a Reetard rim that I've flat spotted, but I'm struggling to find a rim to match the ERD perfectly. It's stated as 540mm on some websites, so I'm looking on that basis until on-one get back to me.

How much leeway do I have with ERD? Can I fit a rim with a smaller ERD without major problems? Is there anywhere that lists rims by ERD? It seems to take a lot of hunting on the Mavic Tech site etc. to find them.

Ideally I'd get something a bit wider – running 2.35" tyres at the moment, which is a smidge over the max for the rim, and have a Mavic 521 on the back which is strong, light and fairly cheap.

Alot of the Mavic rims are around that number, the only two things you need to be concerned about are the spokes being too long and piercing the rim tape/tube or too short and not catching enough threads.

Peel back the rim tape and you will get an idea if the spokes are currently on the long/short side or just right.

There's probs a few mm give and take, check whether the spokes are at the short or long end of suitable length for the rim.

Hmm. Mavic list a "Spoke Support Diameter", which you apparently add 3mm to in order to get ERD.

EN521 is listed as 532, so 535 ERD.
EN321 is listed as 536, so 539 ERD. That said, it's an ounce heavier than the 521, and not welded.

Currently the spokes look like they could be wound in a max of 4mm more. Most are just level with the heads of the nipples. So in theory, it looks like I should have just enough slack to fit a 521 without spokes poking tubes, with it being a fairly deep rim.

Sounds correct.