Everyone wanders at times. It's okay, just bring yourself back to the book.
I sometimes step through the tangerine door as I PR, it seems to help me.
You might consider modifying the chant. I do all the time. Try 4, 3, 2, 1, I'm absorbing this book. That little additions should help keep you focussed on what you're doing.
I used to add "relaxed and alert" to my chant. When preparing I found so easy to relax I could drift off to sleep. I'd yawn like crazy and my eyes would water. Adding the "alert" solved my problem. I also noticed that as I used to PR some of my muscles would tense. By adding the "relax" to my chant, I noticed my muscles would immediately relax. They would tense again, but after a few cycles of the chant, they remained relaxed.
Oh, here's one of my current favourite additions to the chant "I'm the worlds best PhotoReader". This always brings a smile to my face and I feel it helps the absorption of the material.
Good luck and have fun.
Iam2 