Back in the day of being a Fire Suport Man (enlisted FO) we used mils to calculate deviation corrections to the observed fall of shot in relation to the target.
Our binos had a reticle with an index every 10 mils. In practice, we rounded distance from observer to target to the nearest 1000 meters, then used that factor to multiply by the measured deviation in mils, to determine a correction in meters we would send back to the FDC.
For example, target 2800 meters from observer gets an O-T factor of 3. An impact observed 20 mils left of target, would be multiplied by 3, for a correction of "Right Six Zero" (60 meters)
Our compasses had a mil and degree index. A full circle had 6400 mils.
Here's the part where it goes to hell, a circle with a radius of 1000 meters, has a circumference of something like 6283 meters....
