you're always given a starting point in the questions, if nothing is mentionned you assume ISA conditions and you start from 15°c at ground level and decrease every 1000ft.
Now if the temperature is let's say -23°c @ 22000ft and you have to work out the t° @ FL390, you start from -23°c and substract 2°c /1000ft up to the right level.
Bare in mind not to go lower than -56.5°c (if in the Gen Nav or Instrumentations exams for example), i.e. if they tell you the t° @ FL250 is -40°c and ask for t° @ FL390, it's going to be -56.5°c, not less
. I remember a couple of questions like that during these exams
Mcgoo,
They have removed the formulas for NGM in the new CAP edition, I know as I have it in front of me for my Flight Planning exam
Happy New Year!