...but in the exam you have to save as much time as possible...
If you know your stuff, you have plenty of time at the exam. I don't want to sound arrogant, but you don't study for the exam, you study for your whole profesional life, if "pilot" is the only profession you ever learn. Invest a little more time and try to understand what you are learning, then you won't need rules of thumb and mnemonics. (As an instructor, I am really fed up with those students who can tell you for every question if the answer is a, b, c or d, but have no idea at all what the question is about.). Sorry.