Maths books and books with complex formulars to learn always take longer than regular books to read anyway. They are not books per se. They are pages of exercises for you to practice on and become familar with. The benefit of photoreading them is that the formulars and explainations are placed into the inner mind before you attempt the exercises. You may not notice that you understand them better and find them easier to do. Unless you tried doing them without photoreading and happened to find it takes longer to get the whole concept. Even then you have no real way of comparing the impact of photoreading the maths book until you skip the process.
Pr the entire book and then PR the chapters you need to work on a day or 2 prior to them being covered in class or when lecturer informs you that the topic will be covered and asks you to "read" up on it. Remember to PR the 2 chapters before and 2 chapters after the key chapter. As Youngprer said treat each chapter as a book.
Remember that photoreading can help you reduce the amount of time you actually 'read the exercises'. The 'doing' part always took longest with a maths text book. While the photoreading process can help you 'get the concept' faster and help you to 'do' the exercises a bit faster, the doing part is still is going to take up most of the time.
Additionally use the photoreading step of activation in 20 to 30 minute sessions. Giving yourself a 5 minute break to stretch after 30 minutes of doing math exercises and a longer 15 to 20 minute break after every hour.
Alex