More Cold Weather Altitude Correction

There is more to it. The facility elevation must be included in the equation. An undated FAA Advisory Circular gives the following formula:

h(15-to)
C= -----------------------------
273+to-.5k(e+h)

h=height above facility (AGL)
k= standard temp laps rate (.00198deg C/ft)
e= facility elevation
t= facility temperature
to= facility temp adjusted to mean sea level (to=t+ke)
C= Altitude correction value