Reciprocal headings, add 200 minus 20 or vise versa or add 2 minus 2 to keep it simple
150 add 200 = 350 minus 20 = 330
300 minus 200 = 100 plus 20 = 120
That is simple??
What's wrong with.....
150+180=330
and 300-180=120!
Some of these
work-arounds are more complicated than the original!