Use the mirror and after you get the reticle centered, loosely mount it on the rifle and then bore sight it to see how close your rings/mounts are holding it to what you are seeing through your bore...
The more centered you can keep the reticle on a mounted scope the better...I've done this with all my rifles/scopes and get extremely good accuracy and it seems to make cranking the turrets more consistent when returning to the initial settings..
