You need to be at a true altitude of
15,000ft above the QNH datum,1003hPa.
The difference between standard
pressure (1013hPa) and QNH (1003hPa) is 10hPa and at 30ft/hPa this is
equivalent to 300ft.
This means that we need to be at
15,300ft above the 1013hPa pressure datum.
The difference between pressure
altitude and true altitude is 4ft per thousand per degree of temperature
deviation; 4 x 15.3 x -15 = 918ft.
We need to be at an indicated
pressure altitude of 15,270 + 942 = 16,218ft