Originally Posted by Mrs_Often
You can manually set the orphan and widow values to 0, so that a paragraph will not start on the new page even if only one of the lines of the new paragraph would fit on the end of the page (or be left on its own on the next page). You have to set this each book separately:
Either have Calibre do it for you during the convert, by adding extra CSS code. See attachment.
That'll fix the annoying gaps
However, depending on your font and lineheight settings, there will still be a relatively large space between the text and the bottom of the screen, because the bottom menu space is always reserved and is rather big. If a letter that goes below baseline (like g or j) would only touch the top of the menu line the whole line of text would move to the next page. So, though the widows and orphans set to 0 gives a MUCH nicer screen fill, it's still not optimal in all cases (and even varies per page).
Fab - thankyou!
Such a shame you have to do it in Calibre individually each time - will start stuffing it in