SLIDE 15.1. It’s important to know the above characteristics of great circles.
SLIDE 15.2. In using sight reduction tables for great-circle sailing, the table’s declination coordinates are used for your destination’s latitude and longitude. Now H.O. 249 only covers declinations to 30°, which means your destination’s latitude must also be under 30°. Moreover at these lower latitudes the difference in rhumb-line and great-circle sailing diminishes; so it means that H.O. 249 isn’t very useful for great-circle sailing – so we use H.O. 229.