Top of the rim to the underside of the arch is a good indicator, but not foolproof.
The car would need to be on level ground, as you mentioned, both rear tyres would have to be the same in good order and inflated to the correct pressure in both. Then you stand a chance of having it level.
If however the measurements do not correspond then it could be indicating that the spring on the lower side is suffering fatigue and would need changing also. (always in pairs) Hope this helps
