Yeh, had fun and games with bubble levels... these ones in particular...as you tighten them up they're fine right until the last nip, then they kick out of true (on my walthers). I've had to shim them to keep them true.
What I use is a level like this http://www.leveldevelopments.com/pro...section-vials/
which I check against a good level like a builders level first. Then I sit that on the action, and get that level. Only then do I bolt a level to the action checking it agrees with the one resting on the action. I then use a plumb line to level the scope up. This is all done while the gun is in a rest.
I've found the sportsmatch ones which screw directly into the mount more reliable.