This is one of those questions where your probably going to get a lot of different answers . But IMO James has it rather right .
While its often considered more important for the shot gun shooter . It also relates to the rifles itself in that its just one measurement of many .
I think one of the reasons that there are often so many opinions is that the vast majority of folks have never had the opportunity to shoot a rifle that’s properly fit to them . Basically they learn to shoot the rifle they have by modifying their stance and hold to accommodate the ill fit of the gun .
Simply put its something that they just have gotten used to .
So you see the tall , long armed person , wrapping themselves around a rifle like a CVA or traditions , with a short pull and shallow drop , like a monkey humping a football , while trying to get down on the sights .
Or visa versa the shorter built person with their head way back on the stock because they can reach the longer pull of the gun.
Really not much of that maters if the person does a lot of rested shooting . But if that person does a lot of off hand shooting or snap type shooting with a rifle then IMO its preferable have the pull considered in the overall measurements of the stock .
Myself I do a lot of off hand shooting . Frankly I really enjoy long range, off hand at distances of 150-350 yards . Thus I use a very erect stance with a 90 deg shoulder . I don’t want to have to hunt for the sights . When the rifle comes up , I want to be looking right through them . In other words , they immediately center to my eye . Same goes for the pull . When I rise the rifle I want that 90 degree angle of my upper arm to elbow . IE no drooping elbow or where its forward as with to long a pull or back with to short a pull .
So in so many words , the rifle must come up proper so that I don’t have to lean down , search for the sights or adjust the gun to my shoulder .
Basically just the same as a Trapp shooter who wants their gun configured to properly fit the correct stance for what they do . I also think we often forget that a lot of the purpose built shotguns of today , also carry rear reference. IE the shooter may run a double bead or a higher raised rib . Its also not uncommon to see stocks built with not only adjustable butt plates so as to lengthen the pull but also adjustable combs and adjustable cheek plates just like on tournament type rifles .
But again to relate back to what James said . Assuming that taking a single measurement and thus defining the length of pull on a rifle , without consideration of the other measurements really doesn’t achieve much . In fact in some case as with working with a pre-carved stock and then cutting its pull down to fit a smaller person , can result in a long arm with a butt stock that’s nothing more then what one would see on a blanket type gun .
But to answer your question , when I measure a person for length of pull on a new stock . The measurement is from the first joint of the finger to the inside of the elbow. Thus if they lay the rifle on its side so that the butt plate is in the crook of their arm , the trigger finger is right on the trigger .
From their I adjust the measurement to them . IE some folks like just the tip of the finger on the trigger . Are they shooting with heavy cloths on ….. Some folks have longer upper arms and shorter forearms ….. So that first measurement really becomes a base measurement and not something that’s set in stone