I'm working on my own lrs tool ( OS X ) but I can't work out how the page count is calculated for the XML tag. How are you guys working it out?

I think it's supposed to be an approximage number of pages with the default style settings:

A.5.17 SumPage
The SumPage element specifies the number of “View”s (visible pages for the BookSetting element conditions) of the content.

I'm pretty sure it's ignored by Reader's renderer.