to the OP - as analytical as you talk, you certainly lack any intuition, if you could not find the flat spot on the action ( the mag cutout ) - just set the dismounted action on a builders type level resting at the mag cutout - that gives a level action, then use whatever method you care for to level the reticle ( I always line it up with the garage door header across the street )
I can appreciate your comments on intuition, and I have been relying upon my (quite good, actually....according to many, many observers) intuition for many years to help me figure things out.....BUT having been around the block a LOT of times (and not the same block over and over, either), I now realize that, while intuition can often be a quick and helpful way to solve a problem, it can also give you an answer that gives you warm fuzzies, that looks and feels right, but is
WRONG...sometimes just a little wrong and sometimes COMPLETELY WRONG. Sadly, in a small percentage of instances that wrong answer can cost you injury or your life, but most often a wrong answer only causes you a red face. IN THIS PARTICULAR INSTANCE, I have the luxury of this NOT being in any way, shape or form time constrained or an emergency, resulting in my having lots and lots of time to look, measure, calculate, ask questions, get opinions and then apply all of those, tempered with logic, to find the (hopefully correct) answer to the problem at hand, not to mention possibly learning some things along the way.
It is not my intent to try to embarrass you in this forum, but if you don't believe me when I say that intuition can give you some VERY wrong answers, no matter how right they seem, PM me with your phone number and I will give you some examples.
Oh...and while the magazine cut out MAY be square and perpendicular, And it CAN BE SO, I've never seen any guarantees that is always true, given stock pressures, manufacturing tolerances, varying designs for whatever reason and etc. and I have not checked to see if it is in this case or not. Apparently you missed the post where I plainly stated that I have not yet gotten off my (lazy) butt and closely examined the thing by taking it all apart. Rather, I have tried asking a bunch of questions of many people in various places and hopefully getting some thought-provoking answers. (Which I did, BTW)
I, like you, use the buildings around me to check for correctly oriented recticles, but I use mostly the vertical edges and/or corners (which I do ASSUME are truly plumb) on them, thus taking out of the picture any possible error induced by perspective if my position is not 90 degrees to a horizontal line on a building.
Anyway..thanks for your ideas and suggestions!