I think the per page strategy is way too arbitrary. What is a page exactly? A unit of measurement of text? How big is a page? What font must you use. What about technical books and books with pictures and diagrams? If we invented modern electronics before books we wouldn't care about pages. Measuring time spent would be a lot more useful and harder to game. What if the book is dense with so much good information that I feel the need to read slower or reread a paragraph?