1023 -1013 = 10hPa x 30 = 300ft.   Altitude in ISA conditions = 25,300ft.

 

            FL250 ISA  =  +15ºC – (2º x 25)          = -35ºC

                                                            OAT     = -50ºC

                                                            Dev.     = -15ºC

 

Difference between true altitude and pressure altitude is calculated as:

 

4ft per1,000ft per ºC ISA deviation = 4 x 25.3 x 15 = 1,518ft

 

True altitude = 25,300 – 1,518 = 23,782.        Or use CRP5