I think there's an error here in the case of a very tall notebook. The paper claims that there is a finite limit to how far to the right the bookmark can go. But it seems to me that for a sufficiently tall sheet of paper, the optimal fold is at a 45 degree angle with the left edge of the paper folded right up to the top edge. This gives you a length of D - 1 hanging to the right, where the width of the paper is 1 and the height is D.