Thanks Soldier, you are da man!.
The math is quite simple once you have the missing link, which, for me was your equation [1].
Reviewing my research I saw the equation before, but
- In a different form: TAS = EAS*SQRT(AD@MSL / AD_ALT) and then the definition of EAS introduced dynamic pressure or Mach number and I never got results at least close to yours.
- With a remark it is valid only for low-speed flight (~0.3 Mach)