BabyBear
My first equation is based upon ACTUAL temperature and pressure - temperature is easy (use the OAT gauge), pressure is also easy (use the altimeter calibration and adjust for the subscale setting - correcting to 1013.25 hPa/mb). The rest are standard values. So, I'd stand by my max 3% error for airspeed measurement and you could remove that by using the PEC chart in the manual.
My second equation is exact, so long as you have an exact air temperture to work with. There's a small error due to humidity, but it's going to be around the 4th significant figure, so I'd really not worry about it. Altitude / pressure / density do not affect the speed of sound in air, only temperature.
MIKECR
Your ATPL notes formula is correct, but just a simplification of the universal formula I made. Being really picky, it's just "Kelvin" not "degrees Kelvin". Obviously Kelvin = degrees Celcius + 273.15. "degrees centigrade" = "degrees Celcius".
G