May not be all that scientific but have to agree.
As with anything, if it "looks" right and feels right then it probably is. Trust your own judgment, the human eye and the ear are powerful tools for the shooter/gunsmith.
I Level and clamp my rifle in the verticle position, using a level on a top flat. Then I take my best eyeball set that I can. I put a light reference pencil mark on the scope and rear ring. I cant the scope left and then right and eyeball it again. I then look at my marks to see how far off I am. Most of the time, I'm right on. Once put my small level on the top scope cap and it showed level. However, when I eyeballed it, I could clearly see it was canted. So you can't trust this step.
Another point to consider is that in a hunting enviroment, most of us don't check to see if we are holding the rifle in the perfectly verticle position but the crosshairs tell us where we are or should be. I would say that most of the time we are not canted or very little. Now, bench shooting is a different situation.
WSM MAGNUM: I`ve been eyeballing it too Pahoo. But after I get the scope snugged down and eyeball it again, the darn cross hairs still look canted at different times.
To me, that again is your eye telling you that you are not there and to readjust. I have had scope rings turn me out of true when i tightened them down. When Tightening down my rings, I alternate each screw; one at a time and a little at a time.
Just like anything else, We all do our best, in our own best ways. ..
Be Safe !!!