I have not done it that way, though it sounds like a decent method of leveling the body of the scope, though that doesn't necessarily actually level the reticle/dialing mechanism with gravity. I level off the feed rails, then mount a level to my barrel to match, then take it to the range and use a plumb bob to level my optic mounted level with the reticle to get close, and then make sure the scope/optic level is pretty close to matching my rifle level, then if I plan on dialing for vertical adjustment, I do a tall target test to dial in level. What really matters in leveling depends on how you use your optic, they are as follows -
If you use a holdover reticle type method, and don't dial, then the reticle must be plump with gravity in relation to your optic mounted level. Align your reticle with the plumb bob, and adjust your optic mounted level to where it shows level while the reticle is aligned with the plumb. Call it a day, that's all you need to do for it to be perfect!
If you dial elevation, than your internal vertical adjustment mechanism must be plump with gravity in relation to your optic mounted level. This takes a little more effort to do it 100% correct, as the reticles don't always align perfectly with the mechanical adjustment, especially with some of the very functional, but also budget built long range scopes now days. After you use whatever method to level the optic, or get it in the ballpark, the only 100% positive way to get perfect level (to my knowledge) is to shoot a tall target test, I do it at 100 yards as wind and such have less of a chance of affecting my results. Mark a line and ensure that it is perfectly strait and level with gravity when you shoot, and have your aim point centered on the line at the bottom of the target. With a zeroed rifle with an accurate and repeatable load, dial up at least to the maximum amount you would practically dial. Ensure your referencing your level for every shot, and shoot a group, and see where it lands. If your group is perfectly centered on the line, congrats! You did it perfect. However it is very often that you will find it a small amount one side or another, lots of reasons this can happen, especially if dialing 30, 40 or more MOA. At this point, I loosen my level and adjust it the proper direction/amount to make the correction. Shoot again and confirm, and continue making small adjustments until your groups land perfectly on the line, then the parts that matter are 100% plumb with gravity, and that aspect can be completely taken out of the equation.
I apologize, I am rambling at this point. 0400 hrs on my first graveyard in several months.....Sorry ha ha.
