Paging a XML paragraph data

Hi folks,
After rendering an XML file through XSLT we need to page break after EXACTLY 40 lines. This has been done if the elements only contain one line of data... but.... The HTML "span" fields are a fixed width. The problem comes about when the paragraph elements are parsed, some may only contain 2 lines of data or 300. We are using DOM with javascript. Any ideas on how to get around this? A working example would be much appreciated. Cheers Craig