put a level 30 feet away. make sure its level....
set gun up level with some sand bags or sumthin...
look through the scope and adjust to the level 30 feet away.make it where you can see the bubble. i can usually see the difference in the scope if the bubble is leanin one way or the other and adjust to be more exact. i dont have any flat spots on my rifle to level it that way. so i just eyeball as best as i can.
i am likein the mirror idea though and will try that some day.