If you want an "accurate" AFR meter, you need an industrial unit that can read AFR, and not just O2. Problem with just O2, is that the generic units just aren't accurate enough.
In another post I was talking about the unit that I recently borrowed from work (many thousands of $$$) that show AFR to 2 decimal places, as well as %O2 and Lamda.
Problem with O2 reading is that if you are zero, you still don't know what your AFR is. If there's no oxygen, there's no oxygen.
Tim