The truth is that pull length isn't all that important.. That ought to stir up a hornets nest!! Most people can successfully adapt to a pull length from about 13 to 13 1/4 all the way to over 14 1/2. They may have a few problems and some who are not in the normal size range may have more trouble adapting but almost all will find those pulls usable.. But and it's a big one.. they would do much better with a pull correct to their physic.. Include in that wish list of dimensions caster/chamber, rake angle, comb hieght, and perhaps even a bit of castoff or caston as the case may be. The rule I usually adhere to is that the elbow should be close to a 90 degree angle when the weapon is mounted. That is subject to change depending on the build of the person involved. Factory stocks do well for most people, but a well fitted custom stock is almost always going to work better. All the criteria won't be covered here, books have been written about the subject. Also experience is the best teacher. Fit is also more important in a shotgun than a rifle as a shotgun is pointed, not aimed. The rear 'sight' then becomes the face/stock fit. A shotgun that won't hit for someone can many times be corrected by altering the butt stock..