Funny, I was just writing some code to do this earlier today. I lifted this from somewhere on the web, and it is very accurate. You need to consider OAT to get an accurate TAS.

Quote:
I've used a formula that takes care of the PA / DA difference and introduces a temperature correction.
Was given to me a long time ago by an old retired navigator and it's surprisingly accurate.
Here it is :Â*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 correctionÂ*
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.
Unquote

Given:
IAS in Knots
MSL in Feet
OAT in Celcius

ISA = 15 + MSL/500
TAS = IAS + (IAS/60000) + (ISA/500)

I think Matt's point about 1.4% and 2% is very valid, and he meant to be conservative, and don't bank on that gap being safe. Given that it changes with OAT (albeit not by much) don't push VNE when up high.