Good fit is essential, Youngun. The Brits have this down to a science, using 1/32" increments, and lots of them shoot very, very well. I'm not sure we have to go to those lengths. Here's a rough and goodenough guide.
Different parameters apply for "Serious" and the myriad forms of recreational shotgunning. I'm going for a GP fit here. Do this with a weapon KNOWN TO BE EMPTY, for obvious reasons.
First, forget that stuff about if the butt is in your elbow and your trigger finger reaches the trigger. Like most myths, it's true sometimes.If you're 5'9", 160 lbs with a 33" shirt sleeve, it's probably true.
Mount the shotgun and hold your position. If there's room for two or three fingers between your nose and the base of your strong side thumb, the length is close.If not, adjust it and try a few fast mounts to make sure.
Next, focus on a fixed point, like where the ceiling and two walls meet in the corner of the room. Close your eyes, and mount the shotgun. Open your eyes. If the shotgun is pointed precisely where you were focussed, it fits. If not, make it so.
One of the nice thing about repeaters over doubles and singles is that most can be adjusted very well by shimming between the receiver and stock. People use toothpicks, pieces of old hulls, cut up credit cards, or my favorite because it's infinitely adjustable, folded aluminum foil. If the pattern's low, add a shim at the bottom of the receiver,etc.
Next, focus on the point again and see if you're lokking right down the bbl. On a GP shotgun, you should be "Flat" down the rib, not seeing much if any rib, just the bead(s). If there's two beads, they should be superposed, or stacked in a figure 8. If you do see some rib, it may shoot a bit high for you. I like them this way. Some don't.
Now, set up your patterning board. Use a 4'X4' piece of paper with an aiming point in the center. Using your ammo of choice, take a few shots and see where the major impact area is. If it's centered or a bit high, Huzzah! You're almost done.
Next, put up a new piece of paper and using a fast mount/fire, see if the POI changes. If so, move things around until you're centered.
For "Serious" use,or for use in cold conditions, the stock's better off if it's a bit short, rather than long. Otherwise, you're all set.
Finally, if this is not helpful, seek the help of a good fitter. Some folks just can't do this themselves, tho most can.
HTH....