Just set the gauge to factory defaults and you should be good to go.
Because you are dealing with two different volumes of fuel, the percentages are not going to work properly.
Once you have time in your boat, you can reset your levels to a more satisfactory level, though do not change the percentages over to gallons.
The Gallons reading only works on the Cougar FTD models.
If your two tanks seem to prefer one tanks reading over the other, we do not suspect it is a level issue, more of an issue relating to the height of the two tanks being different.
This could be adjusted at various levels to match your preferences after you understand which tanks style you like, or don’t like. Then if you don’t like it, you can return to the default settings.
BCB