After you fill-up (select More, Fill-up on your Scangauge), it will tell you how many gallons it has calculated and you can change the amount in 1/10 gallon increments (based on how many gallons you just pumped into your truck). As you change the amount of gas, you will see the offset percentage change. Remember that percentage. the next time you plug in the scangauge, quickly select Fill-up TWICE. The second time you should be able to adjust the percentage to match the number you determined previously. (You won't need to do this if you don't move the Scangauge around between different cars)

I was getting very accurate results, but the last time I was quite a bit off. I think that part of the problem is that the pumps at different gas stations don't always stop at the same time. From what I've read, the most accurate results will be achieved by always going to the same gas station and using the same pump. Personally, I don't care THAT MUCH about accuracy. After a few fill-ups, you should have a fairly good idea of what your offset should be.