TAS is the most important value when flight planning, because this is the "Speed" at which you are traveling if there are Zero winds aloft.
Lots of typ-a-holics here, I'll try and keep it simple. Three step plan to getting GS. I avoid CAS calibrations merely on the fact they are minimal at best, and your performance graphs in the POH are already set-up so it won't be detrimental to the cause.
IAS: Basis for all of your information.
TAS: Built off of IAS and Altitude(corrected as mentioned above){POH will have a TAS value for your given altitude and power setting}
GS: Taken from your TAS and corrected for direction of flight vs. winds aloft