Although degrees are the most common measurement of angle, official SI units are radians. 
A radian is the measure of an angle that, when drawn a central angle of a circle, intercepts an arc whose length is equal to the length of the radius of the circle. In other words, radian is the angle made by taking the radius and wrapping it along the edge of a circle. The sector formed in this manner will have all sides with equal length. There are 2 ? radians (6,29 radians) in a full circle.
Since all sides of the sector are the same length, an angle measured in radians can easily be converted to distance. To make the calculations easier, one thousandth of radians, mRad’s or mils are used. One mil (mRad) approximately subtends one metre at a distance of one thousand metres.
One full circle consists of 6,29 X 1000 = 6’290 mils (mRad).
The formula to calculate distance (or the size of the object when distance is know):
D = distance or range to the target in meters
S = size of the target in millimeters (known width or height of the target)
mil = number of Mil-dots (mRad)