What we would do when we realized that there was no way we would have the book read by book group was:
1) Finish the chapter you're on, if any.
2) Read the last chapter.*
3) Subtract the last chapter completed (use zero if you haven't started yet) from the number of the penultimate chapter to get the number of chapters not yet read.
4) Divide that by two (lop off any fraction) and add it to the last chapter finished from the front of the book to get the chapter to read next.
5) Read that chapter. That leaves the book divided into two unread sections.
6) For every unread section, divide the number of unread chapters by two and add it to the low end of the gap.
7) Keep dividing the unread sections into more and more, smaller and smaller sections.
The idea behind this ridiculous exercise is to familiarize yourself with the book as evenly as time allows. You get a feel for the whole thing, not just the front end. When you finally have to pinch it off and leave, you will have read some from the beginning, some from the end, some from the middle, some from the middle of the second half, some from the middle of the first half, etc., etc. The idea is to not come off as a slacker or an idiot, although, when our group was larger, I went a couple of times not having read any of the book, didn't tell anybody, and I don't think anyone even noticed.
. . .
*She always read the last chapter first no matter what, whether it was a novel or a textbook or whatever else. She was frighteningly bright, but a little strange, I always thought.