EDIT: Ha! Bob beat me to it
Hi Mark,
We measure the true direction of the flight planned track from a midpoint on the line because straight lines on a WAC or VNC chart are actually great circle tracks (a VTC is different though - a straight line on one of those is actually a rhumb line). For the distances we fly, following a rhumb line track is a lot easier and yet is close enough to the great circle track even though it isn't really the shortest distance between the two points. Flying a rhumb line track is easier for us since it means we simply hold a single heading for that leg of the trip.
The rhumb line direction we need can be found by reading off the direction at the midpoint on the track - midway between the departure point and the destination. As I said, the difference between the rhumb line and great circle track is very small over the usual distances we fly. However, you can still see the difference by measuring the true track direction at the origin and compare it to the true track direction measured at the destination. Depending on how far apart the two points are, you will notice a slight difference between the two readings. If you measure the true track direction at the midpoint you will find it lies between the readings you got at the origin and destination and that "average" between the two is the heading you would need to fly to follow the rhumb line track.
It won't be much of a difference but airlines for example do take the difference very seriously especially for the longer distances they cover. Flying a rhumb line track from Tokyo to San Francisco would be a much greater distance than the great circle track between the two points. The crew could hold a single heading the whole way but it will cost a lot of extra fuel and time.
Flying the great circle track on the other hand would mean the heading needs to change continually whilst en route but at least the aircraft flies the most direct track to the destination. Check the textbook on page 3.1.5 for a discussion on the difference between rhumb line and great circle tracks.
To set up your plotter, it depends a bit on the type of protractor you are using. The principle is the same though. Place the grommet or centre of the plotter on the flight planned track at the midpoint between origin and destination. Rotate the protractor so its guidelines run parallel to the meridians of longitude. See where the track crosses the degrees scale around the edge of the protractor and that gives you your true track for that leg of your trip. Have another look at 3.1.6 to see the steps.
Understanding map projections and the effects on rhumb line and great circle paths is a bit beyond the CASA PPL syllabus but it is still a fascinating topic and amazing to think these charts were already being produced way before the days of any kind of computer.
Cheers,
Rich
