To answer the OP, if they want the answer from CAS, do it on your whizz wheel: Put the Cal Airspeed on the Pressure Altitude and read the Mach No in the "Mach Number" window.
If they want the answer from TAS, in the True Airspeed section, put the OAT on the Mach Index arrow, then read Mach and TAS on the main scale. Alternatively, use the formula TAS= 39M x Sqrt (OAT°C +273.15)
You can use the two methods above to calculate the IAS for a TAS without worrying about the Cr value.
Local Speed of Sound and Temp/RAM rise are irrelevant.
Originally Posted by NTA
the difference in CAS to TAS is not that much
Hmm, that's not right. At ISA sea level, yes, at 40,000ft, almost half.