Refer to figure.
True altitude calculation from a Flight Level:

We are given a pressure altitude (in the form of a flight level, FL80) and we need to calculate the corresponding true altitude.

The first step is to compute the pressure correction by considering the deviation from the standard mean sea level pressure of 1013 hPa: 1013 hPa - 997 = 16 hPa. Since the barometric lapse rate near the surface is 30 ft/hPa, the pressure correction can be calculated as follows: Pressure correction = 16 hPa x 30 ft/hPa = ­­­480 ft. This value needs to be subtracted from the pressure altitude as per the rules below:

Indicated altitude = 8000 ft - 480 ft = 7520 ft.

Note: There is no mention of an ISA deviation, and as such we cannot calculate a temperature correction.

The distance that we have to climb to this altitude is 25 NM. Since we are travelling at a GS of 160 kt, this equates to 25 NM / 160 kt = 0.156 hr = 9.37 mins.

The RoC required to reach 7520 ft in 9.37 mins is 7520 ft / 9.37 mins = 802 ft/min

RULES. The following rules should be considered for altimetry calculations:

• All calculations are based on rounded pressure values to the nearest lower hPa.
• The value for the barometric lapse rate between MSL and 500 hPa to be used is 30 ft/hPa as an acceptable approximation of the barometric lapse rate (if not provided by the question).
• To determine the true altitude/height, the following rule of thumb, called the ‘4%-rule’, shall be used: the altitude/height changes by 4% for each 10°C temperature deviation from ISA.
  For simplification: Height correction for the temp = 4 × (ISA DEV) × Indicated alt/1000 = _____ ft
• If no further information is given, the deviation of the outside-air temperature from ISA is considered to be the same throughout the whole layer.
• The elevation of the aerodrome has to be taken into account. The temperature correction has to be considered for the layer between the station (usually an aerodrome) and the position of the aircraft.

DEFINITIONS.
Pressure Altitude: The altimeter indication with standard pressure (1013.2 hPa) set.
Indicated Altitude: The altimeter indication with local QNH set.
True altitude: The actual altitude of the aircraft above mean sea level.