At the risk of not answering the original question, have you considered using one of the LO extensions to simply export your original document as an EPUB rather than convert with Calibre ?
I have found these can create css styling that is closer to the original source document and easier to understand.
There are two main contenders :
Luke's Writer2Epub add-on
and
Export to EPUB, part of the Writer2xhtml export filter extension from
http://writer2latex.sourceforge.net/ (though this needs a tweaked version to run with the current LO). This creates css styles with the same name as the styles used in the original LO document.
BobC