Using the same Boeing graph from the last thread, I estimate about 20% increase in fuel going from empty to full. This assumes there are 375 passengers on the full flight, average weight of 200 pounds including carry-ons, checked baggage and food, flight of 5,750 miles, and no cargo.

Considering that the graph is only an approximation, the 20% estimate agrees pretty well with the 15% estimate. I'd be willing to believe the pilots without argument.