[chuckfourth]

Interesting debate Im having with myself here.
Actually something I missed earlier, when a fire mission is called in from a battery of the smaller calibre guns the shells fall over two turns anyway so their is no need to worry about the fraction of the time taken for the shells to land, the fall of shot can be adjusted exactly onto the unseen target on the next turn.
So
If anyone thought this was a problem the solution might be to let all the shells that fall over two turns fall in one turn. Or let the same amount of shells fall on one turn, have no shells fall on the second turn but increase the number of times the battery can be called to compensate. And the shift fire mission button would still need to be greyed out unless a new fire mission is being plotted.
Alternately once a fire mission is called there is no adjustment of the shift fire mission button allowed until all the shells have landed (ie the shift fire mission button is greyed out).
A creeping barrage which is what I believe the shift fire mission button is for is still possible, you just need to let all the shells for each call fall before you adjust the range. ie the the shift fire mission button is only available for "new" calls.
Best Regards Chuck.