Or, strike a vertical line on a four foot length of butcher paper set at 100 yards. The line must be plumb. Put a target dot on the line for your aiming point. Start sending rounds downrange maintaining the same aim point (the dot) while dialing the impact up on the scope for yardage. Guarantee (based on your information) you'll watch the impacts move progressively right. If it does, the scope is out of plumb. No need to mess with lapping the rings with quality rings.
Check out the Hollands anti-cant bubble when you get it dialed in.
http://www.sinclairintl.com/optics/optic-accessories/scope-levels/holland-s-signature-series-scope-levels-prod42468.aspx
It really helps with consistancy in the extended ranges. The 700 yard target at my range I shoot at has a slight tilt. Most shooters don't know this and tilt the rifle without knowing. Pisses them off when they move back and forth between yardages and have to keep adjusting for windage on a calm day.
I matched my anti-cant with my scopes reticle with then used the anti-cant to level the scope to the rifle. My elevation tracks true. At least to 800 yards which is the max at the range.