This is exactly correct and it is the most important thing to remember. In particular for courses marked with only one line it is important to measure as straight a line as possible from turn to turn, because it is often difficult to walk along and know what the inside edge of the course is. If you go look at a single line course the evening after (or even the following morning) a race you can see how far athletes strayed from the line. Stakes or poles at the turns really help, so long as they stay in place. (Last year we hosted Olentangy Orange for a dual meet. Some of our kids warming up knocked over one of the poles, and moved it, decreasing the length of the loop by about 40 meters, twice. It took many of our kids until late in the season to beat those times.)Can't say I have an answer about the best method. But, by rule, the course should be measured on the shortest distance. This was a rule change a couple of years ago. Used to be the middle of the course.