Bring up map, full zoom in.
Then, take a quick peek at the field layout.
As amazing as it seems, the icon placed on the flight map is actually correctly represented in perfect scale - which means, if there is a tower right in the middle of the field, and you correctly aim right in the middle of the zoomed up icon, the guns will hit the tower.
So...
* Check the field layout.
* Check the approximate position of the target on the field
* Switch to LAND MODE on CV guns(LAND MODE automatically follows your click and aims correctly for it)
* Click the position which corresponds to the location of the target on the enlarged field icon
* And immediately after you click on the map and guns stop moving, press "F" to fire.