1033 -1013 = 20hPa x 30 = 600ft.   Altitude in ISA conditions = 20,600ft.

 

            FL200 ISA  =  +15ºC – (2º x 20)          = -25ºC

                                                       OAT          = -40ºC

                                                       Dev.          = -15ºC

 

            Difference between true altitude and pressure altitude is calculated as:

 

            4ft per1,000ft per ºC ISA deviation = 4 x 20.6 x 15 = 1,236ft

 

            True altitude = 20,600 – 1,236 = 19,364.        Or use CRP5