Sorry for having overcomplicated things in the following messages...
The equation you would use was derived for a compressible medium and the only value Mach number can take in an incompressible flow is zero.
First of all, I don't know why you're saying air would be uncompressible ? It is very much compressible when you squeeze it into a syringe or any kind of air pump or even a balloon. It also suffers waves of compression when sound travels through it. But even if we admit that LSS would be infinite in an incompressible flow :
Mach number is speed/speed of sound (TAS/LSS)
You're saying LSS is infinite in uncompressible flow... And you're probably saying that at low speeds the air is uncompressible.
Well, problem is sound does take time to travel even in still air. Even in water, a fluid that's almost uncompressible, sound takes time to travel.
So basically we have air at low speed : it's almost uncompressible so we can use Bernoulli's theorem. It is a tiny bit compressible so we can use the sqrt(gamma R T) = LSS formula. If speed increases, the LSS formula becomes "more correct" (according to you, you still have to explain why it would less correct at low speeds), but Bernoulli becomes less correct. And that's where the correction factor is needed.
But in any case, nothing prevents the engineer from performing computations that do not make perfect physical sense. One wants to compute a mach number as TAS/sqrt(gamma R T) ? No one can forbid him to do so.. Maybe tell them they're wrong but nothing more.
This is probably the sort of reasons why EASA ends up asking surprising questions like the one we saw.
Such a scheme may work at low speed and low altitude but it is inconsistent and unnecessarily restrictive.
It is probably the the same sort of reason that could lead to an EASA question like the one we saw yesterday.
Because except by using wrong formulas, I don't see why this 0.94 thingy would be required.
Correct equations allow us to forget about this kind of correction factor.
I was simply explaining the wrong physical reasoning leading to require a 0.94 correction factor.