This question of line-height is finally
solved.
The solution lies not with calibre but with OpenOffice.
Before sending the file to calibre to convert, I have to modify my text in OpenOffice by selecting a
proportional interline value of 120% for the whole text.
After this, the conversion goes smoothly, and my average page in the epub is 22 lines, which I find pleasant to read. (130% gives 17 lines, which is a little too much space)
Les than 120%, you'll see appear a little blank space created by the footnotes.
Up to now, I keep only two extra.css rules:
.endnote {font-size:10px;}
body {font-family:"Times New Roman", Times, serif;}
It seems though, that for the big files, I have to tweak a little to be sure to get the true TNR
serif (see above)