-3/ Position of the claim should be taken in consideration, not only witness : an aircraft shot down 10 km in our lines should be easily accorded, even without flying witnesses.
+1 on this.
Programming-wise like a +15% to the confirmation chance for every KM the claim is behind your lines.
Or that is too complicated too implement perhaps a flat 30-35% added to your chance if the claim is in friendly territory.
