Dave M,
You mention mixing different shot sizes;
If you are shooting at sitting targets, (stationary) it may pattern well, but for flying targets, you will have a much longer shot string.
Even with one size of shot, you still get a shot string. in other words, all the shot does not arrive at the target at the same time, so on a moving target, a bird or beast can escape through a hole not seen when we shoot at a sheet of paper. (Some of the shot may arrive before or after the target has passed)
I must admit, I don't understand the poor patterns we often shoot these days, but Daryl loads and shoots well with thin wads between powder and shot, as recommended by Col. Peter Hawker. (Wads a third the thickness of bore diameter) and apparently does this without a thick wet wad up front.
Re good patterns;
A gun, even a M-loader if well bored, should be able to shoot a 40 percent pattern at 40 yards.
For the sake of it, lets say 30 yards instead! At this range, it should put at Least 40 percent of the pellets in a 30 inch circle.....Drawn After the shot is fired!, and averaged over numerous shots.
Good patterns are judged by how many 5 inch open areas there are. 5 inches is about the vulnearable are of a partridge or pigeon.
The least amount of open areas the better.
a pattern with just two or three 5 inch open areas with no shot marks in them, is usually described as an excellent pattern.
An easy way to look for these, is to make a wire loop 5 " in diameter, and move it back and forth over your pattern on paper.
The old Eley Shooter's Diary had a world of information re. patterns, effectiveness and max ranges, etc. etc.