Rich
All of the above, including the data you've been given (presumably from a very dated textbook) and your stated understanding of the basic principle, is complete tosh!
Fright Level's explanation would have been correct ten years ago, but that was before the introduction of RVSM between FL290 and FL410.
I would suggest that you should actually read Rule 34 of The Rules of the Air Regulations if you want to understand the quandrantal and semicircular rules. You can find it in Section 2 of CAP393, which can be downloaded from the CAA site under Search Publications.