There is nothing definitive or exact about shotguns. Back in the day, when the Germans sought to be precise about patterns, they had "100 field targets." It was given up as being "impractical." More correctly, it was a waste of time.

For a while, the "central thickening ratio" was touted as being the magic key. It wasn't, but it made for interesting conversation. You'll also hear a lot about 60 / 40 pattern distribution. (60% above the front bead, 40% below.) They never are. However, a lot of folks seem to like them, even though very few people could say where the center of a 60 / 40 pattern is at 40 yards.

Pattern percentages can vary up to 20% or more, shot to shot. Is that next shot going to be a 40% pattern or a 60% pattern? The December, 1927, American Rifleman gave summaries from L. C. Weldin (ballistics engineer, Hercules Powder) that showed pattern percentages from 19-95 percent with the same load and choke based on 4,000 patterns. In the October, 1946 American Rifleman a difference of 20% between minimum and maximum pattern percentages was tabulated.

Oberfell & Thompson (1957) starting going on about "patchiness." They had no idea what they were talking about, but even John Olin (the owner and CEO of Winchester/Western) fell for the "patches" theory. Of course, you have patches when the whole pattern itself varies by 10, 20 or 30%. If it falls into "normal distribution" there can be nothing abnormal about it. If that next pattern has 20% less pellet holes in it, the idea that there might be a couple of patches shouldn't be a real blazing shocker.

One recent test (2013) was using factory 1-1/4 oz. #2 1400 fps steel loads through a factory modified choke, fifteen shots were fired through the same gun. As for pellet count in a 30 inch circle, the lowest was 96, the highest was 132. The most efficient pattern was 37-1/2 percent better than the worst. Just what precisely will the next pattern be, the one that you actually throw at a bird?

Even with the very best shells and choke tubes, 10% variance is typical. Add non-spherical shot, soft shot, pellets of various sizes (some welded together) and pattern consistency plummets. In addition, there is velocity variation from shell to shell; the SAAMI standard is +/- 90 fps. Most factory shells are better than that, but not all.

We don't even know the actual velocity of factory shotshells. It isn't possible to apply exactitude when nothing in shotguns is truly exact.