FEELER GAUGES DO NOT ALWAYS WORK
For a nice high end optic that has rigorous QC testing like nightforce, it will work every time.
My Burris 1-4 and Triijicon TR24 were BOTH OFF. Normally this would be an appropriate time to send them in for repair/replacement, but since neither of them have target turrets and are meant for "set it and forget it", I figured who really cares.
Using the feeler gauge method, they were both noticably off.
All of my AR's have free float tubes that I level to the receiver. During install, I tinker with them until the rail shows the same reading as the receiver.
When I mount my scope, I hang a plumb bob line. Crank the scope down as low as it will go. Level the rifle in a rest by placing the level on the free float tube infront of the optic, and then line up the reticle to the plumb bob line. This can take some time seeing as how everytime you touch the scope, you need to relevel the rifle, but after a few tries you will get it perfect.
Its also key to know that your eyes view optics differently. After leveling my TR24 (triangle/post), if I view it through my right eye, it will appear to be canted ever so slightly in one direction. If I view it through the other, it will appear slightly canted in the opposite direction. This is a case of "gotta trust the equipment" and just give your OCD a rest
Even with my nightforce optics, I still confirm them using the level and plumb bob line.