FLEXPWR,
You seem to be only taking into account the actual "horizon distance" for a given FL at a given position, which already makes a significant 20 minutes or so difference.
But.... if you're doing about 600 mph TAS westwards, by the time you've reached the "sunset position" you calculated for your earlier position, the earth will have rotated also, and the sun won't have set there yet...
See my earlier post about Concorde.
While written "tongue in cheek", the scenario is perfectly real.
Even at the equator the rotation of the Earth is "only" about 1000 mph.
So at 45° N or S, it's "only" about 700 mph. So if you're flying westwards at 600 mph TAS, sunset will come at a considerably later time.
And, as said, in a Concorde at 1350 mph... it won't come at all until you land at JFK.
CJ