How about, 1025-950 = 75 hPa at 27 ft each = 2025 ft and then subract 1% of ht for each 2.5 deg C different to ISA in this case 5 deg thus less 2% giving us -40.5 ft as colder therefore lower leaving 2025-40.5=1984.5 ft amsl. Cannot get to my old notes at present.