can't take credit, courtesy of leupold
RR

Adding to Ridge's/Leupold's trick...

Grab a cardboard box just a touch shorter in length or width than the scope body tube (meaning it straddles the adjustment housing but is inside the eyebox and objective bells). Cut two V notches in the mouth of the box, making a cradle in which to rest and roll the scope. Makes centering in the mirror a whole lot faster - and using a mirror is a lot faster, less frustrating, and more accurate than counting clicks (not all models actually have the adjustment screws centered).