Here's a copy of a post I wrote as a reply on the ArmA forums (which was quickly buried). For those who would enjoy firing artillery manually, here's a quick diagram of the aiming reticule and how it's read.
The middle blue line is your current altitude in the artillery piece. The closer line is your altitude + 1000m, and the further line is your altitude - 1000m. Therefore, the distance between each red hash mark equals about 200m. By reading the topographic map, I can see that my target's elevation is around 135m. So if I aim with the first red hash mark after my current altitude, the shell should land within 5m (since that hash is the 140m mark).
I fired the round when the second-hand struck 12 o'clock on my watch. The round will have a 43.16s flight time (41.8+(48.6-41.8)/5), so should arrive two seconds before the second-hand strikes 9 o'clock.
Observing the target area...
Kaboom. (Watch reads exactly 43 seconds)
Accuracy was within 5-10m, at near maximum range of the M119.
That truck is toast.
Another good thing to do is setup a game controller to manipulate the artillery piece with. This way, you can aim using the larger map (see first picture) instead of that pansy minimap. This will make you much more accurate. You still have to close the map to actually fire, however.
God, I love artillery.