I am going to echo what KY Jon, Drew Hause, Crowley and others have said:

The biggest mistake people make when it comes to gun fitment is to think that there is some set of stock dimensions out there that are the perfect numbers for them for all time on everything in all situations.

In actuality, the best stock dimensions are a compromise of various attributes, and change as the shooter's mount evolves, the type of gun changes, the shooting/hunting situation varies, etc. In other words, what works best for shooting overhead driven birds will not be best for walked-up pointed birds will not be best in a duck blind will not be best in a layout goose blind, not to mention the many different clay games, etc.

Of all the major stock dimensions we worry about, LOP is the least important and most flexible. Put another way, whatever LOP a person shoots best is the best LOP under those circumstances. Hewing to some rule like "half the length of you index finger" or "hold the gun in the crook of you elbow" as if that is the absolute best way to determine LOP probably causes more harm than good because it closes the shooter's mind to trying different LOPs to see what works best for him. Although the ambiguity is frustrating to a new shooter, you just have to experiment with different LOPs to find what works for you. For proof of the variability of LOP, look at all the different LOPs used by the various good shooters in Drew Hause's post.

The bottom line on LOP is that it is a balance between the ease (or comfort) of mounting a shorter LOP gun vs. the improved pointability of the longer LOP gun (ignoring the obvious problems at the extremes).

Examples of LOP choices: 1) if you "aim" rather than "point" your shotgun, you will probably be more interested in a comfortable LOP rather than a more pointable gun; 2) As your mount improves, you can better handle a longer LOP gun and thus can take advantage of the improved pointability of a longer LOP; 3) Generally, the lighter a gun, the easier it is to mount and thus the longer the LOP can be; 4) If you are shooting premounted, ease-of-mount becomes less important, and the shooter may choose to use a really long LOP to eke out every last bit of pointability. None of these are absolutes, and the variations are practically endless.

I am 6'1", 220lbs, wear a 16x36 dress shirt, and am far from being an expert shooter. For now I have settled on a 15-3/4" LOP for my ~6lb 16ga SxS with double triggers for hunting, but I am pretty much satisfied with any LOP in the 15" range.

JMHO