Ray, to me, you don't have that much weight to call for that thick of footing. I'd say 8" would be plenty,but I'd tie the footing into the wall with rebar. Figure out where your block will start and stick elbow shaped rebar in so that a good 6 inches sticks out of the concrete. Plus, if there is dirt pushing against the entire 6 foot of the wall, I'd make the footing wider and not as deep. Say, 8" deep and 3' wide, then tie it all in with rebar. That way in order for the dirt to push the wall out, it'd have to take the 2 foot part of the footing that's behind the wall with it if you have 4 inches in front and the 8 inches the block will be setting on.