Well, here is a test book (The Picture of Dorian Gray) modified with a simple page-map. I just make each chapter a page, and named them: (empty) the cover, i-iii the title, contents and preface, and 1-20 the chapters. So there are 24 pages; in ADE the Preface is "iii (4/24)" and Chapter 10 is "10 (14/24)" (you can edit the "iii" and the "10").
How does it work in the Opus or in the Sonys?
(By the way, the "page-map" attribute makes the ePUB
invalid.)