Setting a scope up correctly can be a much harder job than one might think. Getting a scope installed at the optimal eye level for proper head positioning, with proper eye distance, with the crosshair directly over the bore axis, and the crosshair plumb, with necessary droop compensation, with windage compensation if needed, with the turret adjustments near the optical center when the gun is sighted in are a lot of variables to solve. I can share my following experiences:
Many average Joe's can just eyeball a scope well enough for hunting and plinking, job done.
Scope turret caps frequently are not square to the crosshair. Ditch those cute cap levels.
Actions (and their flat surfaces) are frequently not square to their scope mount holes or grooves. Your expensive machinist .005" level is not valid.
Barrels frequently are not square to the axis of the action. Frustrating to find out when you sight in.
Actions are not always squarely mounted in the stock. This will really make your eyes seem crooked!
Scope bases, mounts, rings, etc have varying tolerances. Sometimes you don't even get what you pay for.
In the case of long range shooters or say Field Target where the pellet trajectory has to be precise from 10 yds to 55 yds without any windage error the set up is critical. I have found this tool a huge time saver sometimes saving literally hours on a critical scope set up.
https://www.brownells.com/gunsmith-tools-supplies/sight-scope-installation-tools/scope-reticle-levelers/vertical-reticle-instrument-prod6097.aspx?avs%7cManufacturer_1=EXD%2bENGINEERING The EXD Engineering instrument, Burris Signature Zee adjustable rings, padded vise, and a plumb bob can get it done right (with patience).
Will Piatt