Back in the early1990's when I was starting my gun collections I often swapped, replaced, upgraded, and traded Scopes.
Always wanting to start with the scope reticles factory centered I never liked the mirror method. I loath holding a scope down onto a mirror trying to maintain a good eye box view and moving the turrets. I also like being able to observe or indicate the tube for run out.
A buddy used it after an accidental dropping of the rifle and and found his to have run-out between the front ring and bell.
So I turned out this fixture on the mill that allows me roll the scope while focused on a small object, the further away the better, I like 100 yds.
I start with the scope positioned as it would be on the rifle and secure it with a cloth covered ponytail band to keep a fumble from launching it to the floor.
I place the cross hairs on an object, in my case a distant mailbox flag and roll it.
I adjust the turrets until the cross hairs are stationary. According to how much turret movement is made determines if I have to re-angle it on the tripod.
I have it dovetailed and 1/4-20 threaded for tripod mounting.
I have had this in process of a patent for some time now and its a murderous procedure. I thought I would share here for those with the resources to make their own.
I used to have some v blocks that I used. Don't know where they are at this point. I optically centered my test scope with them a few years back and put them in a "safe place."