The rule of thumb that has been posted many times on PRIME (and I read in an independent forced-induction tuner's guide book) is between 11.8:1 and 12.2:1. Due to the dynamic nature of air-fuel ratio data points, there will never be a "flat line" of AFR value. You can import the data from your engine management controller.....Split Second, AEM, Haltech, Motec.....into Microsoft Excel. Therein, you can perform a few keystrokes that will calculate the arithmetic average of AFR over whatever RPM range is desired. However, just because the overall average is, say 12.0:1, there will be a few spikes above and below. A couple of spikes around 11.7:1 and a couple of spikes around 12.3:1 generally may not be avoidable. What is very important is to make data logs of the wide-band O2 sensor, and look for an extended "trend" of being too lean nor too rich.....concentrate on these RPM bands.