The nearest exact figure I have for ISA is 954.6mb at 0.5km, or 1640ft. If ISA temps applied you would have to correct for 4.6mb at about 30ft/mb, putting you at 138ft higher at 950mb - 1778ft above the 1013mb datum. Correcting to QNH gets you a correction of 12 x 27mb = 324ft. QNH is a higher pressure than standard so this correction brings you to 2102ft.
You have two figures for ISA temperature deviation. At msl it is minus 5deg and at height it is about minus 17deg. Both figures imply a temp correction that will reduce your true height. The convention we normally use is to assume one single ISA deviation, and I can't suggest how to solve this. However, taking minus 5deg the correction would be 0.5 x 4% of 2102ft, or 42ft. Answer 2060ft. Using an ISA deviation of 12deg would give an answer near to 2005ft, but I don't see how to justify that, it isn't the average, and it isn't the ISA deviation at 950mb.
Maybe I got the height at 950mb wrong or there is some other glitch. Anyone out there got any view
Dick