I've used a formula that takes care of the PA / DA difference and introduces a temperature correction.
Was given to me long time ago by an old retired navigator and it's surprisingly accurate.
Here it is : TAS = IAS + 1 % per 600 ft +/- 1 % per 5°C diff with ISA
For instance : IAS = 97 kt ; OAT = 75 °F or 22° C ; FL 75.
1/- pressure correction : 7500 / 600 = 12.5 %
2/- Temp correction
At 7500 ft, ISA = 0°C --> 22/5 = 4.4
Total correction 12.5 + 4.4 ~ 17 % or ~16 kt
Therefore, TAS = 97 + 16 = 113 kt
If OAT = -75 °F or - 60°C the temp corr is -60 / 5 = -12 %, both corrections annull each other and TAS = 97 kt
Just about everybody is right, you can't have a TAS unless you specify a DA, hence a temperature.