Q's. for Hugh:
Good advice from all, but ultimately what LOP works for you, will be the result of you picking-up several types of firearms and mounting them, slowly and rapidly. Rifles and shotguns will not be the same.
When I'm checking LOP on a firearm that I'm considering purchasing, I close my eyes and mount it smoothly and quickly, making sure that I've got a proper cheek 'weld' on the comb. If the LOP is too short, when I open my eyes, I'll be looking over the barrel or over the sights, as opposed to looking through them, in exact alignment. Too long and I'll have to raise my face off of the comb to see the sights properly.
Somewhere, in one of these forums, one member wrote of making a reasonably simple/inexpensive 'try gun' form, out of plywood, which might be of help to you
Q. Do you currently do any kind of shooting at all? Q. Do you have any friends, that have firearms, i.e. rifles and shotgun, that you can use as a 'try gun?' Another option might be for you to go to a local gun store and try out several rifle firearms, until you've found one that truly suits you in LOP.
Myself, I'm good with any shotgun with a 14" to 14-1/4" LOP, regardless of clothing. On rifles, I prefer them at 13-5/8" to 13-7/8" instead of the factory 'standard' of ~13-1/2" Why? I have longish arms, for my height and I have a tendency to 'crawl' a rifle stock. It's not all simple measurement and anatomy. You will, in time, learn your shooting habits/style and factor it in also.
An ill fitting stock, in a firearm with a fair amount of 'manageable' recoil, can beat the snot out of you in a full days worth of shooting or shooting power-house loads and you'll have the shoulder and facial bruising and black and blue marks to prove it.
Good luck!!