I recently drained my tank the same as you did, I used an elect. fuel pump and connected at the point where the fuel line attaches to the water separator filter. After I was satisfied that no more fuel would come out I then filled the tank, which is a 49 gal. tank - it held 48.6 gal. Fuel capacities can vary a little with the density of the fuel, but apparently, this method will get most of it out.