Use whats works best in any given situation. I was able to cover three buildings with just two UCX. I usually use CAS for ground as most times it’s just another primitive. Buildings I usually break it down into two parts as to whats a facade and what is deco as be it UCX or CAS the math is still equal.
The thing though is no one could come up with whats reasonable for any given situation so you really need to see the impact yourself via load testing what is assumed to be a limit or limitation. Since I usually start with the facade work first, more or less painted boxes, I’ll just take what ever I’m working on and dump the entire load in to UE4, give everything CAS, just to run around testing for scale and even with what would be consider less than ideal I still hit high frame rate numbers.
The way to think of it though is 1k = 1 buck and there is no point in spending 50,000 dollars on a five buck asset which is a fancy way of saying just do the math.