To find MOA (Minute of Angle) you measure the distance between the outer edge of the bullet hole of the two furthest apart shots, and then subtract the width in inches of your gun's caliber (i.e. 7.62mm=.308 inches). This will give you the center-to-center distance of your farthest apart shots, in inches. Mel most likely ascertained his measurements at 100 yards (or maybe 100 metres, 109 yds.)
Hope that helps.
I gather the raw MOA at 100 yards (100 meters is actually about 1.1 MOA). The reason is because when you start going beyond 100 yards, the effects of the environment start to play on the groups and it becomes less of a true measurement of the accuracy of the rifle. After getting raw performance, I like to take the rifles out for practical field use and do the long range unknown distance stuff.
MOA is a minute of angle, that's 1/60th of a degree.
FLEA, the MOA is fixed, it doesn't change (so long as the elements don't start interfering) with range. 1 MOA deviation at 100m is 1 MOA devation at 1000m (again, assuming the accuracy doesn't degrade, of course it does in real life because of the elements but that's not a fair comparison to make between rifles...this is why the ranges are limited like mele said to 100m or so). I'm really reitterating what mele said.
The MOA accuracy is found by trigonometry, 60*atan(group size/range).
So just to clear this up, MOA is not the same thing as group size, it's a standardised measure of what angle range any bullet will fly through (think of a cone from your barrel).