I never quite understood how mil-dot scopes worked. I assumed - wrongly - that it had something to do with the military. Actually, the mil is short for milliradians and has more to do with geometry as with the military.
Ryan Cleckner of the NSSF explains how to use a mil-dot scope to estimate distance as well as how to do the calculations. Ryan was a sniper with the 1st Battalion of the 75th Ranger Regiment. I think when it comes to sniping he might know a thing or two.
The video is about 20 minutes long but I found myself watching the whole thing.