Thought i'd post this here, since it was a big issue with my rig awhile ago. I shot a 3" gp. @ 100 yds., and after throwing a tantrum, started thinking about things. So i checked the parallax, and it was just about the size of the gp. After adjusting the PA on the 3-12X LER, i shot a .75" gp. same load.
Check out Catshooter's comment.
http://longrangehunting.com/ubbthreads/showflat.php?Cat=0&Board=UBB2&Number=15670&page=0&fpart=all