My best group of my life was with my Anschutz 1576 .308 sporter, old style 155 Palma Match, 47 grains of 4064, 210 Match and all bottled up in unfired Federal Gold Metal Match. At the time not knowing any better I subtracted the bullet diameter from the hole instead using the actual size of a bullet hole in the paper but it worked out to .012". There were 5 other groups on that same target, few of which went over 1/4" and that not by much.I was pretty proud of that at the time.
I suppose I could reverse engineer that and come up with a correct number, but too much has changed since then. First the load was too hot for brass life anyway. Second, I've learned that the best 3 shot group means almost nothing, and the average not a lot more. Do you really think that a 1/2" 3 shot group means that you can put the rest of the box into the same hole? The worst group is probably a better indicator of what the gun/load can do long term.
Then there's the sad truth that great 100 yard groups are no guarantee of great groups farther out. I watched way too many "great loads" fall apart and way too many average loads stay together to believe that there is any substitute for shooting at the long distances, if long distance performance is the goal.