For the book stay with it. It takes about 5 to 8 minutes to go through a book that size. If you find you need a break just stop for a breather and then when you're ready turn back a few (10 or so) pages from where you left off and continue from there. If you notice the outer edges of the book while photoreading you'll have it fine. You might find that you put a little more distance between you and the book on these larger ones.
For single pages and computer screen the imaginary X technique is the way to go. You draw imaginary lines from the corners of the screen (or window) your focal point is where the 2 lines (would) cross. Soften your gaze and hit (or hold) the page down or space bar.
Alex