7800ft         7800 – 360 = 7440ft

 

 

 

1013               

                                                     30 x 12 = 360ft      

            1025                                                                                                                                                               

The diagram shows the ISA Pressure Altitude of 7440ft corresponds to an altitude of 7800ft. There is also a temperature error to take account of, 4% of the difference between the height you are at and the datum for each 10º of ISA deviation, 4% x 7440 = 298ft, and this is subtracted from 7440ft to show an indicated 7142ft corresponds to an ISA pressure altitude of 7440ft, in high temperatures your altimeter under reads.

 

It might be worth noting that the temperature error correction is small, and is unlikely to affect the outcome of a calculation like this.

 

For an ICAO Semi-circular on 135°M, you need an ODD level, (levels shown

on any en-route chart), FL70 would be too low, the next one up is FL90