The adjustment factor exists to adjust the SG's measurement of fuel used. It is tied to the fill amount.
Let's say for example that the SG said you put in 20 gallons but you really put in 22 gallons. Then, you entered 22 as the amount you put in; the adjustment factor would have gone to 10%. Then, hopefully, during the next fill you won't have to adjust it at all.
The whole reason for you to enter the amount of gallons pumped is for the SG to calculate the adjustment factor. When it can more accurately know how much gas you used, it can more accurately calculate your fuel economy.
This sig may return, some day.