David,

"0.57mach i.e. .57 x 761=433 mph Tas or 382 kTas.  is this correct?"  

Mach is a function of temperature. The warmer the temperature the faster the
speed of sound. The 761 MPH you referenced is for a temperature of 59
degrees F or 15 degrees C. Since temperature decreases as altitude increases
the speed of sound will be less at altitude.

Lynn Farnsworth
Super Legacy #235
TSIO-550 Powered
Race #44
Mmo .6 Mach