I'm about to sight in my ACOG tomorrow. It's mounted on the flat top using a TA51. After it's sighted in, can I remove it, for whatever reason, and re-install it (presumable at the same location (front to back)) and be reasonable assured the "zero" is at least close (or dead on, I hope)? Or does that blow out everything I will do tomorrow and I have to start pratically all over the NEXT time I shoot??

IT is using a repeat zero mount and none of my ACOGs have ever wandered off zero after removing and remounting many, many times. You'll be good to go!