Many years ago I noticed a problem with regular bookmarks. Quite often I'd open the book and waste some time trying to figure where on the page I'd already read to. One solution was to mark the exact place. That worked quite well but sometimes I didn't have a pen or pencil to hand. Then I realized there was a much better, much simpler solution. Use a post-it of some description and stick not to the top of the page but to the side thus marking not just the page, but also the line I'd got to. Simples!