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.