Ah, finally Grimnir wakes up.
What should I do, when I am deemed unworthy to get feedback to my email even though it was you who asked me for suggestions
If you calculate deviation from the target below minimum range it would make shooting below minimum range even more predictable than shooting beyond minimum range. Remember that the small calibre artillery in WW1 was usually withdrawn when the enemy came close and the defense was left to rifles and HMGs. The risk of capture was much higher than the possible benefit of shooting at such short distance.
Therefore I think that shooting below minumum range should be punished in some way. Combined forces are about "division of labour". Artillery is a long-range anti-infantry weapon. MGs do the job at short-range. It encourages tactical playing.
In my understanding a deviated hit is a ground hit. A projectile cannot hit the ground at any point below minumum range, unless
a) propulsive force or
b) gravity change.
Unlikely to happen. So the only two options for deviation are
a) the projectile frontally hits another target. It directly hits that target. Any miss - even a near miss - would result in option b)
b) the projectile continues its flight on its ballistic curve and hits the ground when the minimum range is reached. In order to prevent power gaming the projectile should deviate from that point.
Personally I think that a procedure of a) and b) is too complicated. Therefore I would simply it: Either you hit the target directly below minimum range or the projectile misses and hits the ground at minimum range+deviation.
Note that artillery was used at short distance in earlier eras (e.g. canisters in Napoleonic times). However, with improvement of military technology the artillery became capable of shooting at longer distances but also less suitable for shorter distances.
And finally a bit of humor. Citing 'The Creation of the Army':
And the Lord looked upon the INFANTRY and smiled, and forgave them, for he understood that of which they spoke. THEY WERE INFANTRY, BY GOD! and the Artillery were, after all, only bloody Gunners.