Smarty editing - top_menu.tpl

I've been searching high and low through the forums and cannot find a suitable answer to what I hope is relatively straightforward solution.

I need to edit the top_menu.tpl file so that when the first ten categories have been reached, another column

    tag is generated so the drop down is not one huge long list that goes off the page but a more sensible, multi-column layout is produced.

    Can someone advise the necessary Smarty edits to the top_menu.tpl file?