Saw, the simplest way I know of, and consequently use, is given that you'll always have a rough idea of what a reciprocal heading should be, then if you add together the first two digits of your current heading (say 150 degrees) then the reciprocal will ALWAYS have the same sum, in this case 330, which acts as a good check to see that it is correct. I say only the first two digits because the last digit is going to be the same by adding an integer multiple of ten to it. Heading roughly south east I know the reciprocal will probably begin with a 3 so all I need to do now is decided 3+what = 1+5. That's not difficult for anyone. That rule works for any heading change that is an integer multiple of 90 degrees, its very useful. Hope this helps.