measuring track
millermilla
The reason that you have been taught to measure your track at the halway point is to find the mean of your 'great circle' track - the shortest distance between to points on the surface. When drawn on the map or chart it is called a rhumb line ( wavy line). It is not necessary to measure halfway on a due north/south track for there are no map making angular errors along a line of longitude. If you check a line of latitude using a straight edge you will notice that the line of latitude is concave to the equator. Therefore the errors are greatest East/West and zero north/south. Draw a track East/West for a length of your ruler, say 100nm. Measure from the eastern end aligning your protractor as normal to north. then do the same from the western end. The track that you measure halfway is the mean of the two measurements.
Remember the Earth is round but your map is flat. It is not possible to replicate exactly from one to the other. Your map though is mathematically arranged to be good enough for relatively short distances but you need to follow some simple rules such as the one to which you have referred.
There is no need to do the same when marking drift lines. You are simply marking a line +/- of track, whether from the start point of the track to abeam your first pinpoint or from your pinpoint to close with the next waypoint, whatever technique that you use. You do so only to help you assess during flight an estimated drift from track, dead reckoning. Again, usually good enough.