complicate
I think you guys are trying to complicate things, most companies use an arbitrary >8hrs business, <8 hrs economy. These are scheduled, published times and not actual flying time.
The map is a good idea but would be difficult to draw because different airlines factor in different expected taxi, hold times etc so, two airlines flying the exact same route could publish very different scheduled flight times. Also flying west to east is usually quicker than east to west. The company could make the policy to include the longest leg, flying time wise would determine the class of flight.
Probably the easiest way for a map with circles would be great circle distances and assume all jets fly at 500mph. Therefore for business class at 8hrs you could say draw a circle 8x500 = 4000miles around heathrow and any cities outside the circle would be business class any inside would be cattle class?