I calculate fuel consumption in the following manner, without the use of a fuel flow device. I perform the same procedure every time I go out. Erroneous fuel consumption can alert you to many potential problems early on and potentially prevent excessive damage to motor/hull.
Top off the tanks prior to departure, reset trip odometer on GPS at departure, record total miles travelled from GPS trip odometer at end of voyage, top off tanks again, divide total miles travelled by gallons used, this gives you miles per gallon without the use of a fuel flow device. Of course, if you are travelling without a GPS that has a trip odometer function, you would need to calculate your miles travelled using a nautical chart.
The coordinates for Billy's Hole can be obtained by reading the first few posts in this thread. To my knowledge, there is no place to refuel between Whittier and Valdez, so you will need to bring sufficient fuel with you.
Good Luck,
Ben