That is what I always do as soon as I get a new scope. You can also use this trick to quickly get a new scope on target without firing many shots. Using the mirror, match the image of the crosshairs and it's reflection on the new scope to that of the old scope. I did this Sunday when I installed a new scope on my .257 and the first shot was dead on at 50 yards, required just a few clicks when I moved out to 100.