2" of woods = 1 hex
So, +1 per 2" of woods is my guess.
I think I didn't state my argument correctly.
Scenario 1: I am in the open, you are in the open. There are woods between us. Count inches of woods, apply modifier symmetrically, everyone's happy.
Scenario 2: I am in woods, but you are in the clear. In CBT hexes it's easy to distinguish intervening wood hexes vs the hex I am in, but in inches, I'm not sure how you would decide how many inches of woods represent "intervening woods" (symmetrical effect) and how many inches of woods represent "woods hex I am in" and thus only affects people shooting at me, not me shooting at them.