I believe that on most scopes the mill dots are only accurate on a single power.
Here is what I learned from when I got a mil dot scope a couple of years ago.
http://www.alpharubicon.com/leo/mildot.htm
http://www.remingtonmilitary.com/art...a%20mil-dot%22
Hope these articles help.
I really like the mil dot system myself. I've done ome good long range shooting with my Rem 700 .308 using the mil dots.