Not even sure how you can dial indirect fire in either, I can see direct.
Perhaps make it sort of like the land-gunner mode on CV guns. You click on a map and it gives you a range and bearing from your current estimated location, and the current estimated location of the target.
You have to manually bring your gun to bear on the target, and the gun will shift due to recoil, just like in real life, so you have to manually re-aim.
The longer you stay in one location (say, within 100yds, to let you move so you're not hitting a building or a tree or something when you fire), the more accurate the estimate of your possition, and your target, thus letting you be more precise in your gunnery.
After, say, 15 minutes, I could see someone being able to drop shells on a hanger, or a cluster of buildings with reasonable reliability.
Maybe give us the Wespe, and the M7 priest?