Just to clarify, as there seems to be some misunderstanding,
I'm talking about when designing a book from scratch. And specifically the paragraph font. The only reason I mentioned Calibre is because it is the viewer that the person making the ebook tested on.
Also, while I welcome opinions, if anyone can give me a solid reason why it's bad to set it, that would be great. Intuitively it makes sense not to touch it and just let the device handle it.
I have found this "starter kit"
https://github.com/mattharrison/epub-css-starter-kit That uses 1.5em for line height.
Also on MobileRead's wiki here
https://wiki.mobileread.com/wiki/CSS_template
it has 1.5em
As well, remember I'm asking about line height for basic paragraph text, not other fancy things. Perhaps new threads could be started if people have othe questions about line height.
Thanks!