The problem will always be "how much is a page"?
EPUB is a reflowable format, it doesn't comprehend "pages". PDF does have pages, but PDF is unpleasant for other reasons.
With a proper Page-map/PageList you can match the page numbers given in the index. But most EPUBs don't have them. Amazon Kindle books usually do have Real Page Numbers though.
Of course the proper solution is to create linked references, taking advantage of the
advantages of a reflowable electronic format. But all too often publishers won't bother with that either.
Most of the attention around ebooks has been in the novels department.
Textbooks are still an unoptimized mess, where few actually bother to be well-formed and really take advantage of the technology.
