I'm way over 40. I live in the USA. I prefer a Mil reticle, but would have no problem with an MOA reticle.
For Minute-Of-Angle:
A minute is 1/60 of an hour. A MOA is 1/60 of a degree. Not Imperial. Not metric.
An MOA subtends 1.047 inches at 100 yards. People often round it off to approximate a 1 inch "Imperial" measurement. That's fine if you can accept a 5% error right off the bat.
For milliradians:
A milliradian subtends exactley 1m in 1000m.
A milliradian subtends exactly 1yd in 1000yds.
...36" in 1000yds.
...3.6"in 100yds.
My typical airgun scope is a 12x mil-dot SFP scope. At 12x power each dot represents 10/12 of a milliradian. That means each dot represents exactly 3" at 100yds, or exactly 1" at 100ft.
I use feet or yards for distance to target. And inches for target sizes. I use 10x mil-dots at 12x. I find this to be the most convenient subtension.
The Athlon FFP scope won't do that, but it does not really matter. In reality, either method can be accurate, but it's good to confirm the exact subtensions that you are using. For critical work, I treat all advertised subtension and magnification specs as "approximations". I measure them on the target range and use those actual measurements in my "dope".
Mil reticles are the most common.