In that sense, in the end it comes down to what's fair: I have written a book, and you're using a chapter of the book. If there are only two chapters in the book, you're using 50%. If there are 10, chapters you're using 10%.
I'll ask both of you, but starting with you, Ms. Kiddell, what in your world view would be fair dealing in, say, taking a book and taking a section of the book and not paying for it? What would be fair in your world view and in the view of the students?