Hi Terry,
For mpg calculations, you don't really need to consider whether the tank was
full when you started, unless you have a very small sample of data.
All you really need is total miles divided by total gallons. Each time you
put petrol in, whether you fill it or not, just record the gallons and the
miles. The more miles and gallons you record, the less it will matter what
state of fill the tank was in when you began.
Regards - Dave.