[jackie_w]

From the device the lowest you can set it is 1.3.

In the device config file (easily edited with text editor) you can type any number you like, but I'd have to take davidfor's word for it that the Kobo still won't actually display any lower than 1.225. (You could also manually edit all per book font settings directly in the SQL database if you really wanted)

If you hard code into the epub css, I think you can set any number you like. The Kobo will then honour your hard-coded setting ... with a caveat. If you've hard-coded the line-height in body {...} then the Kobo will override with its line-height setting. If the epub has a specific body class, eg .calibre {...}, and you hard-code in there, then your hard-coded setting will be used (it will also prevent the Line Spacing slider from having any effect). Hard-coding at the p or div level should achieve the same thing (far too much trouble for me).

To put this in context, I took a standard calibre-created epub and read it on the Glo with your Charis SIL (circa May 2012) font.
At line-height 1.3: 25 lines/screen
At line-height 1.225: 26 lines/screen
At line-height: 1.0: 30 lines/screen (far too cramped for me)

Edit: I should have made it clear that the lines/screen were on a Kobo Glo, not an AuraHD. I would think you'd get an extra 2-3 (?) lines on the bigger screen.