Mil Dot formula for ranging
Here is a direct quote from (apparently) Mel's review of the Weaver Tactical scope ( 3x9, Tactical Grand Slam, 3-10X/40mm, Mil/Mil)->
"The scope has a traditional mildot reticle of wire construction and according the detailed diagram that came with the scope, it has .25 MIL diameter dots which would be the USMC dots, though they are round and not football shaped. All of the dimensions are the typical standard mildot dimensions and since it is a second focal plane scope, the mildots are only accurate at one power, 10x in this case. This does allow the clever little trick of zooming in on a 2 yard tall target, such as a 6' tall man, until 2 MILs covers that target, then look at the power ring and that is the range to the target in hundreds of yards. This is a quick and dirty method of range estimation that works fairly well. Of course, you can cover a 1 yard size target with 1 mil and do the same thing.."