There is a much simpler way, valid for subsonic aircraft, that I still use to check my instruments, as it is easily done mentally.
To get a true airspeed (in knots) from a mach number, take as basis a temperature of -35 degrees celsius.At that temperature, the airspeed is exactly equal to the Mach number x 600. Then add or substract 1 kt per degree, rerspectively above or below your -35° basis.
As our mach reading are presented with three digits after the decimal, I just multiply that figure by .6.
Example :Mach .845 and OAT -45° c.
.845 x.6 = 507 kts, from which I substract 10 kts, as -45° c is ten degrees colder than my -35° c basis.
Result : TAS = 497 kts.
It's surprisingly accurate.