@titanshadow: It just isn't going to work. The per screen page count isn't just the font size. If it was, then it would probably be possible to calculate pages based on this and a page size. But, the books also have styles. Throw a scene break with a graphic in it and you suddenly have less words and lines on the page. You would probably have to render the full book at a particular screen size, resolution and font size to get the page count. And even that isn't enough as each epub reader app or handles the HTML and CSS in slightly different ways. Some are actually bugs, but very often it is different interpretations of the standards and all of them are right.
I'd go with @theducks suggestions. The Adobe page counts are the most common. But, add that you can update the comments using a plugboard. I add page count and word count to my books this way when sending them to my devices.
The alternative is to use a different app. There should be an ereader app that uses the Adobe RMSDK and will show page the counts.
It's probably a pity that no app shows the word count instead of the page count. Though that gets into the argument about what a word is. I'm not going there again.
