@Ken Maltby: Exactly (and, yes ^^).
In fact, with the other "in chapter progress" FR implemented in
https://github.com/koreader/koreader/pull/7807, it becomes even clearer when you show the two widgets:
On a 10-page chapter, you go from 1 // 10 (=> 9) to 10 // 10 (=> 0). Remaining is *always* equal to total - pagenum (i.e., it's what's off-screen).