STO, I think you are hitting on a key point here which is that there is nothing about the stock or the action that can be reliably used as a reference level surface…neither for the purpose of installing the scope nor for the purpose of holding the rifle level. It assumes everything is machined absolutely perfectly and the scope rings are perfectly centered over the rail and the barrel is perfectly straight…but that’s a lot of things that have to be perfect and reality often has other plans.
Fundamentally there are just two conditions that need to be satisfied to eliminate cant error:
1. That the scope’s reticle is properly oriented to the barrel. This step eliminates scope cant.
2. That the gun is held level. This step eliminates gun cant.
To satisfy #1, there are commercial jigs you can buy but there is an easy way to do it that requires no special tools. What you do is set up a mirror at a distance of 5 yards and set your AO to 10 yards. Then look through the scope at your reflection. Rotate the scope until the vertical bar of the reticle bisects both the muzzle and objective bell. Put a small white sticker or little dab of White Out on the muzzle to make it easier to see. Now lock down the scope in the rings, taking care to prevent it from rotating when you tighten the screws. Check your reflection one last time to make sure. At this point your scope is properly oriented and scope cant is eliminated.
Now turn your attention to preventing gun cant. The way you accomplish that is to view a known good plumb line through your scope (a hanging string, for example). Now orient the gun so the reticle aligns to your plumb line, and affix your spirit level to the scope tube so it shows level. From now on, trust the level when you’re holding the gun. Trust the level because your reticle is aligned to the barrel, and the reticle is being held precisely in the line of the force of gravity.