That Midway level looks interesting and should work but have not ordered one as yet. As far as cant, I usually just trust my eye. I put a level on the base or good top flat and then eye ball it. After I have set my relief and scope is just snug, I pick it up and again, Eye ball it. I own three bore sighters and sadly none want to agree with each other. On short barrels, I use my lazer bore sighter. Windage is always good but elevation is seldom right on. :barf:
Now, If I have a front sight on a rifle, I line up a C/L mark on my bore sighter and line it up to the center of the front sight and do the adjustments. I still go back and do the eye ball. I find that I can get close enough to trust it. By the way, the confermation is how the bullet tracks on the paper. I usually track very well or it's back to my shop gun vise/stand.
Be Safe !!!