Yes, this is hard to keep track of even after window resizes which changes how many words appear on each page. To do it I'd need to store info about page position in local storage. which is possible with the extension but might be a bit too much effort. I can't think of a way to do it with the bookmarklet at the moment.
Why not use pushState or store this info in the url hash? Both of these will allow you to save bits of info in the url that can be used to make a nice back button experience.
