Two tankfulls later...I've recalibrated uSec/Gal to 128800000. My
MPGuino now indicates fuel used within 0.7% of the fillup amount. For example, MPGuino showed 16.71 tank gal used and the pump was 16.6. After changing to 128800000, MPGuino said 16.60. Still calibrating but that's close enough for my needs.