Go ahead and fill up your tank today till the nozzle shuts off, then add a little more if you want. Write down your mileage and date on a piece of paper, keep that in your glovebox.
Everytime you add gas for the next few months write down the number of gallons including the numbers past the decimal point, the car's mileage, and the date. You'll have accurate numbers. The amount of gas that spills over the side or "could be added" is minimal when you are talking about many tankfuls.
By the way, I went through a tankful last weekend, mostly interstate at 70mph to 75mph - only got between 25 and 26 mpg. Not bad, but not good. 5-speed manual. For comparison, in my '99 Honda Accord I get 28 to 29 mpg regularly in mixed city/highway driving (also a 5-speed manual).