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