Try
this. You need only modify the values in
blue. Alt is pressure alt, CAS M/S is CAS in meters per second, ISA CORR is the ISA temp. correction, P is the Pressure at altitude derived from P0 / (T0/T)^g0/LR where g0 is reference acceleration due to gravity, L is lapse rate (0.0065 K / meter) and R is gas constant (287.053 J / K / kg). T0 is obviously sea level temp, T is your P.ALT temp, 1/d is sea level pressure (ISA!) over computed pressure at crz alt: you'll need to alter this internally from 101325 N / m^2 to whatever value you have on the day. A0 is sea level speed of sound in m/s, A is LSS in m/s for crz alt. Tt is computed TAT. Output TAS is in knots. This sheet gives you values commensurate with the dashed line in this
graph - ie, don't use for values where CAS exceeds A0.