A/F ratio gauges are fairly worthless IMO, unless you are running a wideband O2 sensor. Wideband O2's are fairly expensive and rather delicate, so it doesn't make much sense to install one of these.
The gauge will NEVER give you a real time view of your stoich, but instead will display a latent "trend" of how your mixture is.
Like I said though, do not rely on the gauge to give an accurate description of your air/fuel. I have one installed on my t-bird, and when I first got it, I thought it was something I HAD to have to make sure I wasn't going lean. Now, it's nothing more than a pretty LED that sweeps from side to side. Dashboard decoration.
Believe what your plugs tell you and not what the gauge tells you.