You don't have to decide where the page break will occur. That's the browser's job. As with everything else in CSS, your job is to provide information to help the browser do things the way you want them done.
In your case, you want to tell the browser not to put a page break inside a particular box. And for that there's the "page-break-inside" element. One of its possible values is "avoid". I encourage you to look at
the CSS2 Reference for more information about pagination; it isn't a simple thing at all.