+1 for oneoldsap on this one, and usually this is the normal step in proper scope mounting. I usually go to my dungeon where neery a sound can be heard, and count clicks, I also use a pen and paper.. I count to 25 and put one talley down at the end of it all you can center your scope and properly mount it and check down the bore to see if their close at say 25 yds. This is also unpractical for autos, unless you have seperated upper from lower... Any way where did you hear about a mirror trick? I used to work all my teenage summers at a busy Gun-shop and have never ever not once heard of that one.. That's why we all love this forum!!!!!
P.S. You had it sighted in why mess with it? The rifle doesn't shoot according to bore-sight it shoots according to your 100 yd adjustment.....of course there's a difference...
Thanks for coming!