I think the easiest method is the meter method.
Figure out how high your target is in meters (man is 1.8-2 meters for example), multiply by 1000 and divide by the number of mills.
So, you have a guy 2m tallx1000=2000 and say he's taking up 3 mills so 2000/3= 667 yards. But PLEASE correct me if I'm wrong because I usually am.
I found the web where I picked this one up, it's here:
http://www.shooterready.com/rangingclass.html