[Flyboy]

I generally propose a 10% limit on arty for PBEM games (includes CAS and attack helos). Keeps things balanced, and avoids massively unrealistic arty components (like 20 batteries of 203mm supporting a single tank company).

Of course, that's for a mtg-engagement. For an assault, I find that 20% for the attacked, 10% for the defender works well.

I guess I'm more concerned with game balance than realism.