Proper fit is a subject for book length discussions. There is no real good short answer to your question.
A proper fit of a SG involves measuring and then adjusting, if necessary, five different parameters or aspects of the stock. LOP is just one of them. The others are: Drop at comb, drop at heel, cast, and pitch.
These measurements are taken by means of a contraption called a "Try Gun". It is a shotgun with a stock that is adjustable in about a bazillion ways - for each of the five aspects.
Its use requires someone who REALLY knows what he is doing.
You go to a range. There is a steel plate painted white and mounted 16 yards away from a shooting point. You are handed the try gun. It is loaded and then you are instructed to look at the target, mount the SG, and shoot at the target. The idea is to get the gun to shoot where you are looking. The bead is not used.
The fitter looks at the point of impact and adjusts the stock on the try gun. You do this repeatedly until the pattern is centered. Then the figures are recorded.
Like all such explanations, this one is an oversimplification.
The simplest way to measure LOP and get pretty close is to take a measuring tape and measure from the crook of your arm, along the forearm to the center of your trigger finger. Done this way, I get a LOP of 15 1/4". The Try Gun put me at 15 3/4". Pretty close.
Pete