Any scope is going to work better and be easier to use, if its vertical crosshair represents a straight line toward the gravitation pull. If that scope has a reticle with reference points for 300-500 yards, having your bullet drop straight through them is all the more important. While I don’t claim to be any kind of precision shooter, I do like to shoot (at) varmints as far as I can reasonably expect to hit them. With that in mind I recently purchased a plain vanilla Nikon Buckmaster, 3×9 with the bullet drop compensating reticle. It was slated to go on my go-to hunting rifle, an old Winchester 670 in 30-06.
The first step to indexing a vertical crosshair is to level the action. Winchester thoughtfully provided flats on the bottom of the action, which I placed on a level and and shimmed until the bubble looked right.
Finding true vertical can be easily accomplished with a plumb line. It’s windy all the time here and a carpenter’s plumb would swing like a pendulum in the breeze. So I suspended a short piece of railroad iron, on bright green piece of 1/4” nylon rope, from a suitable limb.
With the action leveled and the vertical crosshair on the plumb line, I adjusted the scope until the line disappeared- except in the hold-over circles.
Next I snugged the ring screws diagonally from each other, slowly and carefully to avoid disturbing the crosshairs. A final check confirmed this. This seemed like a good time to bore sight the new scope. I began by stapling a paper bowl to a stake, which I drove 100 paces out in the field.
Next I oriented the rifle until the bowl appeared to be in the middle of the bore.
Without disturbing the rifle, I looked through the scope and observed the crosshairs were about a foot away from it. A few clicks of windage and elevation brought them together.
This will just about always get you on paper at 50 yards. From there, it’s a simple matter to fine tune the settings until you’re on at 100 or whatever distance you choose to zero.