Hi sharp,
If you're at an ATC unit, just ask for the ICAO location indicator decoding book. It has an excellent, fold-out map that shows all the various regions and their associated identifying region letters. If I recall correctly, they don't possess any real logic to them; they are a hang-over from the past when the AFTN couldn't handle a lot of figures. I sometimes wonder why plain language hasn't replaced a lot of the data put into FPLs, NOTAMs and the like.
We're stuck with them for now. Cheers.