wolf, if you can tell me how you calculate an aircraft's speed, then your equation makes sense. the fact is, there's no way to do that unless you already know the distance you're travelling. as you're trying to find the distance, you can't calculate speed.
Your almost on the answer.
Many ways to calculate the air speed itself. But modern aircraft use a pitot tube, that measures air pressure. Increased air speed results in increased air pressure.
Calibrate this instrument with ground based measurements and now you have an accurate means to measure air speed. Speed over time gives you distance.
My father has been a pilot before he stuck a gps in this plane and that is exactly how he calculated his distance.
Obviously wind will change your ground speed. But this is not an unknown, and can to a certain degree be calculated before taking off with proper planning, by calling in ground based weather stations on your route. This is generally how pilots used to do it before gps, and they managed to still get to a 2 km long airstrip over 1000km away.
JRowe, as I am not sure on your perspective of the world map, is the map I linked above your accepted map? If not I can do some calculations on your version.