A lot of people "in the know" will say this is not a way to judge your a/f by. It's a "narrow band" sensor and is only accurate when close to stoich. (14.64:1 a/f ratio)
However I've had my wide band hooked up for about 2 months solid. I watch it and I watch one of the stock 02s on my scanguage. And I'll tell you this. It's repeatable and accurate down to about the 12.0 - 12.5 area. Richer than that and there is not much difference... which is really too rich anyway.
I'm confident enough in having compaired them side by side that I've taken the wideband out of my truck and just use the scanguage. Next summer I'll most likely spray like this too.
He who is enslaved to the compass is free to sail the seas