Determine the range with the reticle.
Since we now know that the markings in the reticle have a known dimension, we can use that scale to determine the distance to the target.
In order to do this, you must know the size of the target though.
There are many good online tutorials covering this so I will just suggest that you check out these links:
Insert a list of mildot links here.
In addition to using the formula, there is also a tool called a mildot master.
This is a simple sliderule that allows you to quickly figure the distance without doing any math.
Check out the website and especially the online demo here: http://www.mildot.com/testdrive.htm
There are also iphone and pda apps that will do the calculation for you.
The most important part of "milling" (measuring) a target is to get the measurement as accurate as possible because small differences in measurement make huge differences in the calculated distance to target.
I recommend that everyone keeps a mildot master in their kit as it can be used for both MIL reticles and MOA reticles.
To use the mildot master with an MOA scope, just move over to the right side and use your target size as the "bullet drop" dimension.