I'm no expert with Calibre, but here is what I do. I'm using Calibre version 0.5.10.
In the Override CSS, I put the following:
Code:
p {line-height:2.5}
body{font-family: sans-serif} p{font-family: sans-serif}
I set "Base font size" to 10.5
For EPub, the fonts sometimes don't come out as a like, so in stylesheet.css, I'll do a global replace of "serif" with "sans-serif".
Good luck.