In addition to Dave's comments, a gun doesn't fit you correctly if it slaps your face or is leaving a small bruise on your shoulder (ie. the entire butt isn't on your shoulder).
Because a shotgun is pointed not aimed and the "rear sight" is your eye, think of stock adjustments in the same way we adjust rear sights on a handgun or rifle.
I was fitted in the UK a few years ago and here's how it was done:
I was given was a tightly-choked over/under try gun (an AyA as I recall). This is a shotgun with an adjustable stock that can be moved for length, drop, cast and pitch. The over/under try gun gives you the correct measurements for everything except a side by side. More on that later
The patterning takes place 16 yards from the board. This distance from the board is critical because according to the Churchill method of fitting every 1/16" of stock movement relates to one inch of pattern movement on the board. If your pattern is, for example, 8" too high then the stock needs to come down 1/2".
I was instructed to mount the gun as I normally would and hold it just below the board. Then I was told to swing the gun smoothly up towards the center of and fire just as the bead touched the bullseye. I fired repeated rounds (the board was whitewashed after each shot) and the stock was adjusted until the pattern was going where desired. Then we went to the clays range and shot some high incomers and some minute final adustments were made.
We then repeated the entire event with a sxs try gun. According to my instructor, the sxs requires a different measurement than an over/under or single barrel because of the lateral barrels. The measurements between the two are different in cast, drop and length.
As I recall, the total cost about about 150 pound sterling.
Did it work? When I got home I measured my guns and found a Beretta sporting gun that always slapped my face had wrong dimensions. It was shipped to a gunsmith who trimmed the wood to the correct size. The gun now doesn't hurt and my scores increased. I later ordered a 12 gauge custom sxs from Spain and with the correct measurements it doesn't hurt despite being 6-1/2lbs and not having a recoil pad.
Is it worth it? Well that's up to you. I don't think it makes that much difference if your guns are comfortable and you are shooting pre-mounted games like skeet or trap. The advantage comes in more for hunting and low gun games.
Final point: Just like a suit of clothes you can look good even though the measurements are not precisely on. Same with a shotgun. And like a suit of clothes if you lose or gain weight, stock measurements will also need changing.
Paul