After playing with the rule as written for a couple years now - there are some aspects to this that I do not like; namely - too much of an alternative for artillery to fire at support units (especially heavy artillery) and too difficult for planes.
I think planes should almost be able to hit a support unit as easily as a front line target (maybe even easier), but it should be more challenging for artillery counter battery fire. . .(but all artillery should be about the same effectiveness).
So - I am thinking about changing this to planes can target support units and hit them at their bombing face value & ALL artillery can fire and hit support units on a 3 or less - regardless of their printed value. . .
Let me know what you think. . .