Paul said in the photoreading book and study course that you can set your purpose by asking questions like

"What is my ultimate application of the material?"
"How important is this material to me?"
"What level of detail do I want?"
"How much time am I willing to commit to satisfy my purpose?".

He also indicated that setting your purpose can take a few seconds.
It takes me about 10 minutes to answer all those questions and I still don't feel my purpose is exact with what I really want. How do I set an exact purpose in a few seconds?