Via Inside Higher Ed I learned about a case in which university presses brought a suit against Georgia State over fair use for excerpts in course readers and online course materials:
Her challenge, she writes, is to determine what size excerpts are “small enough” to justify fair use. Here, after reviewing a range of decisions, Evans settles on 10 percent of a book (or one chapter of a book) as an appropriate measure, allowing professors enough substance to offer students, while not effectively making a large portion of the book available.
I guess this is how sausage is made — 10 percent seems like a nice round number, let’s go with that one. By the way, that’s 10 percent including front and back matter, not 10 percent of the text.