Yes it varies, but not by enough to get worked up over. A straight side rim has sides about 1.5mm thick so 3mm does it. OTOH a hook edge rim (most modern HP rims) probably have walls about 2.5-3mm at the hook, so 5-6mm difference inside to outside.

You're not dealing with rocket science, and as the charts show there's plenty of fudge room, so pick a number and go with it.