How to ballpark IAS vs TAS? Is there some reasonably accurate ballparking formula to allow for quick conversions between IAS and TAS as a function of pressure altitude and difference of ground temperature relative to standard atmosphere? That is,

E.g. "for each 1,000ft subtract X%, for each degree F/C subtract Y%" so that if I'm at 8,000ft and ground temperature is 10 C, I simply subtract 8X% and add 5Y% to get a good approximation.

My idea is to have a quick ballpark to start working with, in case there is no time to start fiddling with the flight computer, or in cases where the error would be insignificant for practical purposes.
Indicated vs. True is a hard one, as every aircraft has an instrument error and a position error. Once calibrated into Calibrated Air Speed however, this document gives CAS as a function of TAS:



All of them more or less straight lines, gradients a function of the altitude.

For temperature: the rule of thumb of density altitude can be used. From this website:


  Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F). 


So the procedure is:


Convert pressure altitude with temperature deviation. Ground temperature = 10 °C so density altitude = (10 - 15) * (1000/2) = -2,500 ft
Find TAS at 30,000 ft by subtracting 2,500 ft, then applying the TAS thumb rule of 2% / 1,000 ft: TAS @ 27,500 ft = 27,500 * (1.02/1,000) = 1.55 CAS.


At 30,000 ft and ground temperature of 10°C , TAS = 1.55 * CAS