The QNH is calculated from the QFE reduced to msl assuming ISA conditions which means that there is no temperature error between the QFE datum (at airfield elevation) and msl.
The aircraft needs to be at a true height above the airfield of 15,600 - 1,500 = 14,100ft.
ISA at 1,500ft (airfield elevation) is +12°C. If the temperature is -20°C the
temperature deviation is ISA -32°C.
Temperature error is 4ft per thousand ft per degree of ISA deviation = 4 x 14.1
x 32 = 1,805ft.
The aircraft would need to fly at an indicated height of 14,100 +1,805 = 15,905ft above the QFE (airfield) datum or an altitude of 15,905 +1,500 = 17,405ft above the QNH (msl) datum.