From: Leeuwarden, Netherlands
Hmm I don't know the exact numbers because I'm not a numbercruncher, all I know is what is efficient "gamewise". My experience is that 10 tubes against a soft target out in the open = very efficient. You can see the green readiness bar decrease, and experienced tube units do get a lot of kills as well. That's why Arty units get high exp levels. I'd say 100 Artillery Battle Stack number is pretty solid, so try to aim for that... but there are exceptions that this rule. I have had situations where I bombed an enemy unit up to 250 points, meaning I had a significant penalty but I had the tubes available so why not do it anways. You will see at one point that shooting at targets that are on open ground are much more vulneralbe to Art. fire, units in dense forests or hills are much much tougher to crack, and can take many turns of bombing and encircling before they surrender. So when facing the choice of either bombing an enemy unit out in the open with twice the 100 points (200 Artillery BS points = 20 tubes) or dividing that between two enemy units each with 100 ArtBS points... but one of these units is taking cover in a forest/hills/city... the choice is easy; go for 200 Art BS points on that one unit in the open, it's gonna be toast.
Early on in the game I produce Art units of say 2-5 tubes. Less tubes is very inefficient because they don't hurt nobody, one shell falling out of the sky is uhm... laughable. 5 tubes already make a dent, 10 tubes punch a hole. So later on in the game I recombine those early units into 6-10 tube units. But like I said when facing a very large Inf unit out in the open (and sometimes the AI has masses of Inf coming at you) don't be afraid to use 2-3 of these units against that target. There is no opportunity fire in this game, so unless you want to use the Action Points to move a unit you might as well use the AP to shell that target.
And have them near their HQ as well!!! Art tubes get kills fast (without getting killed like the soldiers do) and so does their exp lvl rise high as well, so their HQ benefits from their exp as well. The more troops you kill the higher the HQ exp gets. The higher your HQ exp the better your forces fight. (that is the more fighting bonus they get).
Sooo long story; when facing soft targets out in the open; one full turn of shelling with 10-15 tubes (if available, certainly not early on in the game) then attack with combined arms and multi angle attacks and the HQ nearby. When facing a well dug in opponent, loads of Inf in woods, hills and mountains... expect many turns of hard fighting, shelling, bombing and encircling... my latest random game reminded me of Italy at Monte Casino, or Verdun, two game years of fighting and shelling, engineers buildig roads across rivers and hills, continuously building airbases to keep the planes nearby.
Oh and heavy Artillery absolutely rocks, especially lvl 2 and higher. They can make a dent even in well dug in forces.