On Friday, October 30, 2015 at 8:27:44 AM UTC+3, Jonathan St. Cloud wrote:
As for reciprocals, plus 2 minus 2, or or the reciprocal works for most headings. i.e., 180 +2 - 2 is 360. Doesn't work for every heading but works for most.

Huh? It works for everything if you don't mind subtracting 360 if the answer is over 360.

Otherwise it works for *exactly* half the possible headings (0 - 180) and +20, -200 works for the rest.