If you believe your stock to be "true" and your rings to be "true", use a bipod and put a level on top of the scope rings bottom half (which should be mounted on the rifle at this point). I like the level app that I got on my iPhone, but any level will do. Now shim the bipod feet (or adjust the tilt if your model tilts) as necessary to obtain perfect level. Next (keep the shimming you just used) attach the scope and point the rifle in such a manner as you can clearly see the vertical edge of a building or plumb line (as long as you KNOW it is plumb) and line the vertical portion of the reticle such that the verticals are parallel to the plumb line (or vertical edge of a building). IE - | |


Tighten your rings to spec and re-check your work...