Relationship of true and calibrated (indicated) altitude:
TA= CA + (CA-FE)*(ISADEV)/(273+OAT)
where
TA= True Altitude above sea-level
FE= Field Elevation of station providing the altimeter setting
CA= Calibrated altitude= Altitude indicated by altimeter when set to the
altimeter setting, corrected for calibration error.
ISADEV= Average deviation from standard temperature from standard in the air
column between the station and the aircraft (in C)
OAT= Outside air temperature (at altitude)
The above is more precise than provided by the E6B or similar.
i think this is the formula i used. it works out if it is