This is definitely NOT an
important question, just one I'm mildly interested in. If there's a fix, great, if not, no matter.
When I switched from a Paperwhite 2 to my Clara last month, I converted all my new epubs to kepubs because I actually
like the reading progress stats bar. So far, I've not read any of the few kepubs I've bought from the Kobo store, still have hundreds of Kindle "refugees" to work through. So I don't know if this problem is unique to sideloaded and/or Calibre-generated kepubs, but I suspect it may be. The stats bar is hopelessly out of whack, bearing no connection to reality.
Here are some screenshots to illustrate:

98% read, over 2 hours left to go?

This was from the 5th "page turn" into a new book, "19% read", apparently. It got me wondering about the page count, so I checked and the Kindle edition of this book is listed as 100 pages, when I turn the header on, it tells me I'm at "page 16 of 60". Since I'd been reading only a minute or two, that seemed unlikely.
So I opened up the other book to the point at which I had taken screenshot one - 98% read, >2hrs to go, and checked the page count.
This seems to explain why the stats bar is useless. If the page count is nonsensical, clearly any data derived from it will be. But is there a fix?