I have mine dialed in pretty well, it is usually pretty close when I fill.
You may already have this but first make sure you have the correct values entered in the Scan Gauge. The engine size is 4.0 Liters. Also make sure the correct gas tank capacity 21.1 gallons
(I didn't believe that our tank was that big when I was setting mine up but I guess they are).
If you want to use the tank gauge you have to tell your Scan Gauge each time you fill up. You may already know but you can set the percent of error each time you fill up by reading how many gallons you put in, and what your Scan Gauge thought you needed. Once I had the percentage right I haven't had to touch it.
I hope this helps, and I am sure most of it is stuff you already know.