Simple is good if it works. Cheap and simple is even better. In the video below, Aram von Benedikt of Outdoor Life shows how to use common, everyday tools to get your rifle scope’s crosshairs perfectly vertical.
The tools for von Benedikt’s method would cost you less than $10 if you had to buy them new. This includes $2.25 for the plumb bob, $3.98 for the level, and about $3 for a small mill bastard file. The first two items were priced at Lowe’s. I just paid $2.95 last weekend at a local surplus store for a Kobalt 6″ file.
So, you can go fancy with a purpose built tool or you can go simple with tools that have multiple uses around the house. The choice is yours.