Fuel usage depends on altitude and speed. If they were flying low and fast, they would not have gotten 2600 miles.
How about low and slow —
I’m assuming the same altitude used to calculate the red and yellow lines in this graph, but a slower speed obviously as the flight path bends east:
https://www.facebook.com/178566888854999/photos/pcb.740971779281171/740971732614509/?type=1&theater
Low enough and slow enough to hit the point where the 811 arc crosses the S20 Parallel — right where they are looking now.