It does not need do be precise. It should only change with every screen / page and give a rough impression of the total size of the book. Therefore it could be word count, page count, screen count, the old metric or (what was called "reference mode" if I'm not mistaken) a metric based on section + paragraph count. Whatever. And since it would be optionally displayed as part of header/footer, what harm would be done?
It does not need do be precise. It should only change with every screen / page and give a rough impression of the total size of the book. Therefore it could be word count, page count, screen count, the old metric or (what was called "reference mode" if I'm not mistaken) a metric based on section + paragraph count. Whatever. And since it would be optionally displayed as part of header/footer, what harm would be done?