I have read in a couple of different places that the F-104 Starfighters best glide with T/O flaps up is 275kts... And they went on to say that above FL500, this is supersonic..

Ok is that true? I am having a bit of trouble buying it.

According to my calculation the speed of sound at FL500 would be

Speed of sound 761 mph X Speed of Sound Ratio at 50K on a standard day.. So 761 X 0.8671 = 659.86 mph

659.86 mph/1.15 = 573 knots...

K... thats not 275 knots... Am I missing something? Whats the deal? Anybody got any sources?