Simple mathematics in terms of the normal aiming. As for calibration/zero after firing, the sight unit is routinely checked during firings. This will tell you if the angle or direction of the barrel has changed and allow you to correct for it.
Typically you will choose suitable ground and fire “bedding in” rounds much like a mortar to help compact dirt beneath the gun Something like a 105mm gun will still roll back slightly, but this is by design to mitigate recoil. A slight delineation is not significant in the context of indirect fires as they are not pin point accurate anyway.
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