Originally Posted by Ripplinger
You can force the Kobo to have tighter line spacing by editing the stylesheet.css file for the epub. 120% or 1.2em works well for me. Once it's within the epub though, the slider to adjust line spacing from the Kobo GUI has no effect, which isn't really a problem if you've set the css correctly to your liking.
The line spacing slider doesn't work even if it can make changes as the minimum of 1.3 is too big to start with and can only get bigger from there.