When you're not sure about how to set up a purpose...think about exactly how you want that information to postively impact you, by acquiring the knowledge in that book, magazine, etc.
Start thinking about exactly how high a level of comprehension will satisfy your purpose and curiosity for reading.
Taking a few moments to reflect on your purpose on about a dozen books in a row will create an anchor, so you automatically approach any printed information with a positive outcome in mind. 