Based on 4 fill-ups, looks like the ScanGauge was 9.3% optimistic vs. MPG calculations done at the pump for two 1.8L 5-speed Civics (both 2007 models).
Now I can go back and adjust the numbers & graph in this thread.
I've been driving with mine set at +10% fuel use, and I think I'll just leave it there. Makes me try a little harder.