Hi Evel the above advise is good. I solved this one by giving each direction a number and keeping track of the two numbers
For example N=-,S=+
and E=-,W=+
So for your example in N/S plain you have-10+20=+10 (ie 10NM up the screen)
In E/W plain you have +10-20=-10 (ie 10NM left on the screen)
Having said that you need a method you are happy with and just thinking about it again has made my brain hurt!
P.S. When you get NE, SW etc all bets are off!!!