Hey VFR, could you unpack that a bit? Give us some worked examples? Seems like a good trick.
Garfly. If you are tracking east, let's say 140 deg just add 2 to the hundreds and minus 2 from the tens =320 deg, if your track is westerly it's the opposite, say 270 then minus 2 from the hundreds and add 2 to the tens = 090. Pretty quick and easy.
vfr