FL150, QNH 993hPa: altitude in ISA conditions = 15,000 - (20 x 30) =
14,400ft.
On CRP5: In ALTITUDE window put altitude (14,400ft) next to temperature
(-30oC).
Go to ISA altitude (14,400ft) on inner main scale and read true altitude
next to it on out main scale: 13,660ft.
On calculator:
Difference between ISA altitude and true altitude = 4ft per thousand ft
per degree of ISA deviation.
FL150 ISA = +15o - (2o x 15) = -15o.
OAT = -30oC. Deviation = ISA -15oC.
True altitude = 14,400 - (4 x 14.4 x 15) = 14,400 - 864 = 13,536ft.