It seems to me you have a general purpose statement that would work. If it doesn't try being more specific. The problem may lie with the activation questions from your trigger words if you are not getting what you want out of the book.
You might want to rephrase your goal of fully understanding the to to be to have an 80 to 90% comphrension of the topic. Mention how you could would like to be able to apply the knowledge you gain from the book.
For example for a book relating to overcomming telephone phobia. My stated purpose was 'I am photoreading this book to gain any useful imformation that I can use to enhance my listening skills, that I can apply in my life. I am also interested in any techniques that I can use to make using the phone easier.'
I find the more specific I am with my intention to find information that I can use to put to use in my life the more successful I am with the material.
Alex
