My turn to state the obvious:
Add 200, subtract 20 (or vice versa).
Example: heading 236
Reciprocal: 236 - 200 + 20 = (036 + 20) = 056
Another: heading 123
Reciprocal: 123 + 200 - 20 = (323 - 20) = 303
Also, you have instruments (DI, VOR etc) with it on, in front of you: just read it off.
Going back underground now...