I learned the best way to center scope reticles from Bill Ackerman of Optical Services Comany. With that procedure you will need either a simple V-block fixture for unmounted scopes, or a rifle-holding vise to immobilize a rifle on which a scope is already mounted. In the latter case, loosen the rings just enough to permit rotating the scope easily, but not so much that the scope wobbles while doing so.
Set up the rifle/scope or the V-block device so the scope reticle is aimed at a reasonably distant (100 yards or so) target or easily distinguished terrain feature. While viewing through the scope, rotate it 180° so the elevation turret is beneath the tube. If the reticle is off center, you will see it slew to a different aiming point from the one you established initially. If that happens, use the elevation and windage adjustments to shift the dot or crosshair one-half of the distance from its new position in the direction of the first aiming point. Now, while viewing through the scope again, rotate it back 180° to its original orientation with the elevation turret pointing up. If the aiming point of the reticle drifts from the prior (second) reference point on the target, once more adjust the elevation and windage controls to move the reticle one-half the distance from its current (third) position. At that point, the scope reticle should be well centered for all practical purposes. Don't forget to retighten the scope rings if applicable. Note from The Gun Guy: If you are using rings with clamp-like mounts having two screws on one side to tighten the ring, you must be careful to maintain your verticle crosshair perpendicular to the firearm's barrel as securing this type of ring will cause the scope body to rotate toward the clamping screws.