As a first step use
--force-page-break-before-tag=h[12]
If that doesn't do the trick, look at the HTML source and try to figure out the correct argument to --force-page-break-before-tag or
to --force-page-break-before-attr
You can check how many page breaks are in the output lrf by using lrf2lrs and then counting the the number of <Page elements. On linux
Code:
grep -c "<Page" book.lrs