I use a carpenters level.
Check the level of a table or counter top.
If there is a slight lean use buisness cards or paper to shim your level.
Then set your rifle on the level. Use the square part of the stock in front of the mag.
the object is to find the square part of your stock.
After that I turn the magnafication up all the way and focus on a plain wall.
From there you just eyeball to your best ability to center the recticle both vertical and horizontal.
I do not know if this is the best method but it works for me.
