Well, the average result of 1d6 is 3.5. So if you want an average unit to be rolling 4d6 with an 50/50 hit-chance, the average Defense ought to be 14. Extrapolation and comparisons are pretty easy from there.

Does decreasing Offense affect the Xd6? I have a bit of experience with wargame-systems, due to the 3.5e Mass Combat system I made (You can find it in my extended sig if you're interested) and that was a difficult issue.