"Oh, fair to middlin'."
A very British reply is "Could be worse!"
Well, there's also "finer than frog hair." I love it when I say that and people do a double take and just look quizzical. "How fine do you think frog hair is?" I will ask. "Pretty fine?" they reply. "I'd say so!" I can then say "So I must be doing pretty fine!"
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.