Have not actually checked it out so do not know if it is of any benefit or not.
I do recall though some years back reading one person's opinion to take the point where you forward hand gripped the gun in shooting & extend a line from there to the center of the butt plate/pad etc. For correct pitch make the butt plate perpendicular to that line.
This would quite obviously show a fat beavertail needing less pitch than a splinter & an O/U needing still less. As I said though I have not checked any of my guns to see how they "Pitched Up" to this standard, have always relied on the standing against the wall method or measurement.
That methodology is wonderfully vague as you pointed out, not to mention a couple other variables that could be introduced! I'm sure that are all sorts of geometric relationships that change as pitch (however it may be described) changes. Unfortunately, wonderful as numbers are, not all numbers are meaningful, useful, or necessarily obtainable.
