I use the EXD device
BROWNELLS : EXD ENGINEERING : EXD ENGINEERING VERTICAL RETICLE INSTRUMENT - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools
which is very well made
I checked it with a level and it was dead on
you set your rifle up in a vise and put the EXD on the scope and barrel and when it is level it means your rifle is uncanted and your barrel and scope are in alignment
While your rifle and scope are aligned and uncanted you need to level your reticle by rotating the scope. I aim it out the door to a 4' level on a fence 30 yds out. When the EXD reads level and your reticle is level with the distant horizontal or vertical object, you now have to carefully lock the rings down. It is much harder to do then you think, carefully tighten the screws from side to side, alternating as you go.
After your EXD is level and your reticle is level then you need to be able to duplicate that at the bench or in the field. So take this opportunity to install a ScopLevel
Scoplevel Anti Cant Leveling Device
that folds down when not in use
Before I started doing it this way I tried the shoulder-it/rotate - shoulder-it/rotate and would get to the range and bench it and get frustrated cause the reticle was off. Then I tried the level-level-level
and found it frustrating and inaccurate (YMMV) cause finding a stable level area on your rifle is chancy at best, turrets on scopes are totally unreliable and the rinky dink bubbles a turn off.
But that's just me.