I've always had an advantage, these last forty or so years, of living where I could have a bench rest close to the house (these last 28 years, front-porch bench rest) and almost unlimited shooting distances. This talk of "variable MOA" with varying distances has never been the case for my toys.
My 1970-vintage old pet '06 was always sub-MOA for five-shot groups. When it and I were much younger than nowadays, I could get ten-shot groups which were barely over one MOA. Ate lots of deer meat courtesy that old rifle. As far as reliability at distance, I had a one-shot kill with the hit within an inch of my desired point, at 350; one other out at 450, about the same, give or take an inch or so. The rest of my deer were within the usual sub-200-yard ranges.
So I finally got off my duff and set up targets at 500 yards. A 1997-vintage Simmons 44Mag 3x10 which I'd put on for no particular reason beyond curiosity. Hokay, same old sub-MOA at 500.
Customary in the world of shooting, the five-shot group is the accepted standard test for a rifle's capability from the bench. It's taken for granted that the shooter is competent.
Standard practice is center-to-center in determining group size.
And, as said earlier in the thread, MOA is the most easily understood term for a rifle's capability. Or should be, anyhow.