You can reset the Scangauge using MORE>MORE>MORE>USE DEFAULTS. One thing I've learned is that after the 3rd tank, you might not want to constantly fiddle with the fillup adjustment.
I found out the hard way that it skews results later, so I zeroed it out (after 50mi and 1 gal) and I'm hoping my numbers are more realistic now that I've gone back to my original adjustment factor (12.2%). I knew something was wrong when I released the gas pedal and my instant MPG wasn't dropping as fast as it normally was and cruising at 45mph/17TPS the instant MPG value was much higher than normal (which inflated my trip and tank MPG) even though the GPH appeared normal. Somehow the SG wasn't accurately adjusting for gas usage. MPH, distance traveled and GPH were still accurate.
I'm not sure if removing it while the car is parked would be a factor.
Next time you fill up, record what your car says and what the scangauge says:
SG tank miles
SG tank MPG
SG tank fuel used
car miles traveled
car fuel used
...and plug them into this calibration excel sheet.
CleanMPG Forums
Hopefully those values will produce more accurate results the next time you refuel.