betaboy,
No, that's not a good idea. The CAS / Mach Changeover altitude is entirely dependant upon Pressure (i.e. Pressure Height) in isolation from Temperature and Density. The maximum achievable TAS for the given Mach / CAS schedule occurs at changeover height. Wind variations can give you you highest GS at virtually any height.
The TAS arising from a particular Mach Number is entirely dependant upon Temperature, in isolation from Pressure and Density.
The only time that Density (as a Density Height) comes into the speed formulae, is in the conversion from EAS to TAS. The older methods of using CAS and Density Height to find TAS actually yields DAS, Density Air Speed. DAS is acceptable with minor error for low and slow operations, but inappropriate for higher and/or faster operations, where eith the 'F' factor is applied to DAS, or, more appropriately, conversion to Mach Number is made from CAS and Pressure Height, and then, using Temperature, find the TAS.
Regards,
Old Smokey