Thank you Pilot Roger,so it is a generic name for a collection of tests.I had thought it was adding or subtracting 180 to a compass heading,silly old me.
I was going to suggest a method I was taught many years ago, of two up, two down and leave the third alone and like wise two down, two up and leave the third alone. Ie 320 put it through the machine and you get 140.Simples.
I can suggest a company called airline prep at gatwick that can help you prepare for tests etc,anyway good luck.