The comment about the reach from the trigger to the pistol grip is a valid one. If you are built with slightly shorter arms what about your glove size? I find that the pistol grip areas on all the rifle stocks I've tried are almost just right for my large to X-large glove size hands. If you're more a medium glove size a wood stock and then have someone trim maybe a 1/2 cm or a hair more from the front of the stock and refit and refinish it would be in order if you're doing a lot of shooting. That would automatically also shorten the overall LOP.
... just went and tried it again. Here's some ripoffs from what I found.
"A good test for a rifle for an individual shooter is to turn the forearm (the person's forearm, that is) palm up, parallel to the ground, then lay the rifle on it with the stock along the forearm, trigger guard outward and the butt firmly against the upper arm. Bend the trigger finger as if firing, and bring it into the trigger guard. If it falls naturally on the trigger, the LOP is OK for that person."
A few other links I found pretty much mirrored that. Some word for word. You'd want it a little shorter if you're shooting with a padded shooter's vest of course.
I'm new to shotguns as well and found a quick google on "length of pull measurement" got me to a site that mentioned how to measure your forearm to find a good starting point.