Purpose should be pretty easy to find. If you just say, "to get the information in this book," that typically isn't compelling enough. I would suggest making it more specific to the book you're reading: "I want to learn how to recognize and apply chart patterns to my stock trading," "I want to understand the world situation that caused World War I," "I want to understand the proper way of exercising using plyometrics to increase my strength."
That said, if you are having trouhle, I'm assuming that you are having one of two problems:
1) You aren't really interested in what the book's about. Possibly you're using it as a "study" book to help photoreading. If you have a book you don't care about, you're not going to do a good job PRing.
2) You're still anxious about the whole program and that's getting in the way. I.e. you're taking it too seriously. If you're worried about being perfect with a given book, you'll experience anxiety. If you're anxious, you won't PR well.
The solution to the first one is to get a book that you are generally interested in. The solution to the second case is to not take it so seriously (i.e. not sweat over it). PR a book that you want to read, even if you won't get all the information you want out of it. If you get anything out of it, it seems that you'll be doing better than you would previously and you'll get better. If you're worried that you missed some information in the book, simply activate it again. Or in smaller sections. Or Rapid Read it. Make your purpose to understand the content of a chapter instead of the whole book.
Hope that helped.
