If you're not familiar with Calling For Fire, read this piece:
Your goal is to bracket the Artillery shells onto the target
To do so, you will send adjustments to the Fire Direction Center using the command panel on the right.
After the first shell lands, you will need to adjust your Left/Right Deviation
Each Hash mark on the crosshair represents 10 Mils (a small Degree, there are 6400 mils in a circle) Since this is an angle, the distance to the target changes the deviation. The distance between the target and the shell at 1000 meters is considerably less than that at 8000 meters. To compensate, you will use the OT (on target) Factor. To get your OT Factor, truncate the distance to the nearest thousand. If the target is 1000 meters away, your OT Factor is 1, if it is 4000 meters away, the OT Factor is 4. Multiply your OT Factor by your raw adjustment to get your final number. If it lands 3 hashes left at 2000 meters, multiply the 30 mils by an OT factor of 2 to get your final left/right adjustment.
Add/Drop is still under construction, for now, each hash represents 10 distance units.