My VesselView allowed me to track GPH and total fuel. I would top off the tank, then reset the fuel amount in my app. I would always allow 10-15% in reserve just to make sure that I could get back to the ramp to allow for any error. I found VesselView to be accurate to 2-3 gallons within what should have been the actual amount of fuel in the tank, so that last part is the 'ballpark usage". I do not think I ever had less fuel than the app said I had, and was always just over at my next fill up.
It would have been nice just to have a gauge in the boat that worked for more than one trip, but I could accurately track real time data using my VesselView in other areas, so it was a workaround.