I don't know if there's a simple rule of thumb for EAS - CAS because it depends on altitude as well as speed.
Remember that at sea level pressure altitude compressibility error is zero even at high subsonic speeds (because that's the altitude the ASI is calibrated for).
Unlike the Mach meter, the ASI does not know it's altitude, so it can only be calibrated free from compressibility error at one particular altitude (which is sea level by convention)
An EFIS does know it's altitude so could theoretically display EAS at all altitudes. In practice most civilian EFIS display CAS in order to be compatable with mechanical ASIs, i.e they deliberatly emulate the mechanical ASI's compressibility error.
If you search the internet you'll probably find a compressibility error correction chart. It is the same for all aircraft.
Last edited by Rivet gun; 17th September 2010 at 16:07.