Does stuff like this really matter?
Does it matter if an algorithm is off by +/- 1000 words or so in a 100.000 word novel? Does it even matter if the page approximation is +/- 10% off as compared to a paper version?
I've been using 2100 characters a page for a long time; 2400 before that, but I found books were too 'short' compared to paper ones, so I adjusted this. As all books are counted the same, one can get a very clear indication of how 'long' a book is, and 2100 chars/page seems to be close enough to the paperbacks I've owned in the past, so I can also approximate how 'thick' a book is.
Personally, I like the count pages plugin fine as it is
Now that I have an original cover for the KA1 that actually works, there's a fair chance I'll be switching to the KA1 as my main reader. I've put the count pages count into the book's title, while the reader counts on its own. I'm now reading a book which Count Pages sets at 302, with 2100 chars/page, while the reader sets it as 341 pages (screen turns, it seems, as the page number increases as I turn a page).
Good enough for me. I can see how far in the book I am on the KA1, in the form of "page x from 341", while the Kindle will show "page x" (from 302, but it doesn't state that), and I can see my custom page count (302) to compare this book to others, even across readers.