I’m not 100% sure I understand the problem so I’ll answer two ways:
If the question is “How can I track (and count) my extra pages as I jump around the book?” there’s a “you might wish you hadn’t learned this much about Beeminder” style answer at Restarting Odometer to non-zero value - #17. This would result in every page you read counting, with some futzing with the odometer.
If the question is “How can I set my goal intelligently if I need to read some unknown number of pages more than what’s in the book?” aka “My Beeminder goal is sort of forcing me to read ahead even though I really ought to slow down and re-read.” then I would say that maybe you should re-frame your goal in terms of the first version. If your objective is to have a coherent understanding of the book (not simply to turn the pages), then framing the Beeminder goal in terms of “Getting to the last page” is Beeminding the outcome not Beeminding the action (per What To Mind: Picking a Metric | Beeminder Blog). If you frame the goal as “Read pages that I need to read at some reasonable pace that isn’t anchored to a specific date or length of book” then you don’t have this problem anymore, although you introduce the new problem that you’re not guaranteed to finish the book by a specific date in time. So maybe it is just a question of which is more important? My suspicion is that returning the book to the library on time is its own motivation and that Beeminder’s role here should be “Don’t forget about the book entirely” and Beeminder doesn’t need to do the work of “Calculate the exact pace of reading required to get it back on time”. But I could be mistaken / useless advice too. 