RE the "mirror method", a question and a few comments.... Doesn't it rely on the outside rim of the objective lense being perfectly square with the centerline of the scope?.... I've checked a few scopes to see how square the front is to the tube, and some are good, some are out by quite a bit.... I have one (a $200 scope) where if you put a Machinists square on the front of the Objective and then measure from the square to the tube just in front of the turrets, the difference is about 0.080" (2mm).... To me, that means that the reflection of the crosshairs would not align with the crosshairs by half that amount (eg. 0.040" or 1 mm) over a distance of 7 inches.... Over 100 yards (3600"), that would be 3600/7 = 514mm or about 20".... That would be about 80 clicks off center, by my calculations.... Not only that, but rotating the Objective may or may not cause the reflection to "walk around" in a circle, depending on if the lack of squareness is in the rotating part of the bell or in the threads that it rotates on....
It would seem to me, therefore, that using a mirror to center the crosshairs "optically" may not be the best method.... If the Objective is not square to the tube, I would think that the mirror, likewise, would not reflect the image of the crosshairs straight back down the center of the optical path.... Counting clicks relies on the manufacturer centering the parts in the tube properly, which may or may not be the case.... That leaves us with rotating the scope in vee-blocks bearing as far apart as possible on the tube as likely being the best method, IMO....
Bob