Where does oval end and oblong begin?

I am preparing facial outline studies for a drawing class and need some clear labels. I can draw the outline, but explaining why I called it oval rather than oblong is harder. I want the notes to describe what is visible rather than suggest that one shape is preferable. I have oval face shape on my reading list for the terminology. Would you mark the widest area first, then compare the overall length with the width? I can add construction lines, but I would like to keep the finished page readable. The jaw and forehead also need separate notes so that the label does not hide their differences. An example of useful annotation would help more than a list of celebrities with similar faces. The assignment asks for a reasoned description, so simply writing a category name will not be enough. How would you explain a borderline example to someone learning to observe proportions?