What ultimately you want is the reticle horizontal with the horizon, this makes sure the reticle is parallel with gravity. So when the bullet is on its flight path, if the reticle is not centered on the bore, the flight path will still be parallel with the vertical plane of the reticle. Most people want the reticle centered on the bore, so they do not have a constant windage built into the windage zero. Therefore giving more balanced amount of windage adjustment, left and right. The way i do it, is i level the scope with my rifle, then i level the reticle with the horizon. I level the scope with the rifle by using a parallel small enough to go between the scope base and the saddle of the optic. Then i use a plumb bob hanging off a tree or sort that i level the reticle with.
