I found this in the Developer section here - the issue being the large amount of wasted white space at the bottom of many pages, which is caused by foolish defaults for widow and orphan prevention.
In calibre, when converting a book, you can insert additional css which can override the existing settings in the book. In the Common Options/Look and Feel, Extra CSS panel, insert the following lines:
It seems to work well on epubs - for the first time, text uses all the usually-wasted white space at the bottom of the screen, and goes all the way down near the menu-bar area. You get several more lines/page and fewer page turns.
BIG thanks to textchimp for this one!
Last edited by EldRick; 12-03-2012 at 06:39 PM.