If you find that 2% / 1000' is not sufficiently accurate, it is more accurate to say that TAS = IAS plus (IAS/60 per thousand feet). IAS/60 is of course just airspeed in nm/min.
If one thinks about it, one will see that 1% per 600 ft is almost the same as 1 in 60 per thousand feet. But I prefer to divide by 1000 than by 600!
Example: speed 180kts or 3nm/min. TAS = IAS + 3x altitude in 000's of feet.
Naturally it still doesn't take temperature into account.
Cheers,
O8