MOA = Minute of angle. It's right at 1" per hundred yards of distance. So, 1 MOA is 1" at 100 yards, 2" at 200 yards and so on.
The exact measurement begins with a circle having 360 degrees and each degree having 60 minutes or 21,600 minutes, total for the circle. Do the trigonometry for a circle of 100 yards--which is 3,600 inches. I ain't gonna.
Bench rest shooting for competition is for five-shot groups. You are not competitive unless your five shots measure less than 0.1 inches in dispersion, center-to-center.
I use sand bags on my bench rest. I tweak and fiddle with bedding of the forarm and with tailor-made handloads so that my hunting rifles provide 1/2-MOA groups for three shots. Some have been pretty good at holding that for five shots. I have an "attitude" that only the first shot is what's really important when I hunt.
In today's world, with the quality of today's machine tools, there are few rifles that WON'T shoot inside of one MOA.
Experience plays its part, of course. I've been loading for and shooting centerfire rifles since 1950. Back some ten years ago I set up a 500-yard range, here at the house. Now, I commonly zero my rifles for 200 yards. With my .30-'06 zeroed in that manner, I have to allow for right at four feet of drop at 500 yards.
Okay, says me, "Play time." I had to guesstimate four feet of holdover and allow for a breeze--so I held about a foot of daylight to the upwind side. My first shot was about six inches low at 5:30; the next was one inch low at 6:00. I figured that wasn't bad for an Olde Phart with tri-focals.
Lotsa guys here can do better...
Art