Traditionaly, one MOA in the USA shooting sports was exactly 1 inch per hundred yards of range. That was based on small bore and high power rifle targets' scoring rings spaced in even inches apart and originated over a hundred years ago
'Twas also based on scope sights with external mounts (Sidle, Lyman, Unertl, El Monte, Fecker, Remington and others) with each click on their 40 tpi screws moving the tube .0002" inch and with the standard 7.2" mount spacing it moved impact exactly one inch at 100 yards. .0002 is 1/3600th of 7.2 inches just like 1 inch is 1/3600th of 100 yards. And metallic rear sights Redfield, Lyman, Gates, Vaver, Warner and others) with their 40 tpi adjustment screws turning 1/3 revolution across 4 clicks moving it .0083333333" which is exactly 1/3600 the distance of the standard 30 inch sight radius of target rifles; 3 MOA per full turn of a knob.
Is this so darned hard to use?????????
A couple of decades ago, this was no longer toloerated by folks who didn't shoot smallbore or high power matches. They had to have the trigonometric values used because they could not comprehend such a simple way of moving sights and bullet impact. But they had no problems with two standards for miles (nautical versus stature) nor mil standards of which there were 4 different ones on this planet.
End of rant.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master