Separation is not quite like pregnancy. It depends upon circumstance.
The terms under which separation should be achieved are defined, and by those terms, it may be achieved.
There is a crucial difference between separation and the assumed minimum miss distance, which will be less for an aircraft under IFR in class A at FL100 where the base of CAS is FL 95 than the aircraft 1000 ft above it.
Spitoon, the relevant CATC lecture hadn't changed when I went through, albeit not quite the glorious 35 years ago that you mention.
Either way, what is the relevance of the OP's question, in the operational world?