[Edited by moderator]
Ironically it just takes changing a value in one line of code in the .css to do the trick compared to the whole tinkering with calibre that you describe.
Originally Posted by dassie
In Calibre, select the book, then press convert. Move to the Look And Feel page of the conversion options, and select the ´filter style information´ tab. In the other CSS Properties field, enter `line-height`. Then press Ok.