Ideally you should maintain an awareness at the back of the head. The chanting helps to stay with the pace and task at hand. It is not unusual for the mind to wonder while you're photoreading and it probably better to be OK with it than to worry about it having a detimental effect on the photoreading session.
However I'm inclinded to bring my attention back to the page turning when my mind does wonder. I found that by holding a question in mind about the book or topic of the book I seem to have faster information from the book. This is works for me and everyone will somehow adapt the process to work for them.
While learning, aim to stay with the book, if the mind wonders gently return to the book, notice the tangerine, remember why you're photoreading the book but stay in the flow. Check what's working, if you find your mind wondering and have no problem activating the book fine. If activation isn't working go back to the basic procedure as taught in the book.
Alex