@paola,
Regarding the calibre3 thing... every time you do a Calibre conversion, Calibre renumbers the
calibrenn class names. If you had an epub with a class called calibre3 then you could overwrite the css for this particular class by doing another epub-epub conversion with
in the ExtraCSS box for this particular book (on the Look&Feel page), i.e. not the ExtraCSS in Preferences.
The caveat is that if you enter something in the individual ExtraCSS it will be used instead of the Preferences ExtraCSS. If you've already forced the @font-faces etc into the epub, it may not matter.
On the other hand it might just be faster to use the Tweak-epub feature to edit the width% in the epub's css.