Using the plumb bob Method works very well, but you have to make sure you have the rifle level first, which creates it's own problem and puts you right back where you were. I use a plumb line sometimes. I have one hanging from a tall oak limb about 20yds off my back deck. It's just a piece of cordage with a 8" pipe wrench tied to it, works perfectly.
All my bolt guns have pic rail bases on them so I usually just use a set of feeler gauges slid under the center of the scope. It's quick and easy, takes less than 2 minutes and it will level your scope with your action/bore everytime provided you have quality components. If your interested in this method I think there is a YouTube video of it.
Using the plumb line method, to level the rifle just install your lower ring halves and set a level across one or both of them and level the rifle, then carefully set your scope it without moving the rifle and level it off the plumb line. Again, as I said before, if the rifle isnt square to begin with, the plumb line is useless.