When you fill, you go something like:
MORE > FILL UP
And it gives you the chance to enter or correct the volume of fuel you actually purchased in place of the volume of fuel the SG calculated you burned. You're adjusting a "fuel offset" which is applied to further calculations.
Note: You can also adjust a speed/odometer offset.
I'm sure you can still get the user manual from their site. Yup:
https://www.scangauge.com/support/user-manuals/