http://calibre-ebook.com/user_manual/xpath.html
You'll probably want to do a conversion with debugging turned on (it's in the conversion dialog, the little ladybug icon on the left -- just pick a folder you'll remember). Do the conversion and then look at the generated HTML file to figure out what element your headings are being generated as. Hopefully you'll have some h3 or h4 elements in there (obviously no h1 or h2 elements are being found or the standard chapter xpath would've already worked). You might find that the elements are div or p elements with a class only used for headings so you could break on that instead.