I had do to something similar when I had a script output text, Latex and RTF files of the same data. To find out the page numbers of the RTF I used the text calculation and muliplied with a factor. It was astonishingly exact

You could just take a measurement of the average size of text and any html formatting that gets used and try to calculate a conservative length. As long as your calculation is more than the real stuff you are on the safe side and the worst that happens is some empty space at the bottom of the body cell

This is especially easy if you yourself generate the html from the data. Just keep a running counter of row and column as you generate the html.