James on "WordPress Category Walker - How To Split Categories Into Multiple Columns?"

I'm attempting to build a mega-menu for my WordPress blog. As part of the menu I'm using a custom category walker to list categories as links in a dropdown div (ParentCat>ChildCats). What I'd like to do is split the child categories into multiple columns so they fill the full width of my dropdown div. I know I can do this by just floating the list elements but I'd like the order to be vertical (1, 2 and 3 in the first column and 4, 5 and 6 in second column etc).

So far I have the below walker that structures my dropdown div and displays the child categories in one column. Can anybody provide any input as to how or if I can split the child categories up into multiple columns? I've seen several articles regarding this but I can't figure out how to apply the logic inside a walker?

class Navigation_Catwalker extends Walker_Category {

// Configure the start of each level
function start_lvl(&$output, $depth = 0, $args = array()) {
    $output .= "";
}

// Configure the end of each level
function end_lvl(&$output, $depth = 0, $args = array()) {
    $output .= "";
}

// Configure the start of each element
function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0) {

    // Set the category name and slug as a variables for later use
    $cat_name = esc_attr( $category->name );
    $cat_name = apply_filters( 'list_cats', $cat_name, $category );
    $cat_slug = esc_attr( $category->slug );

    // Configure the output for the top list element and its URL
    if ( $depth === 0 ) {
        $link = '<a class="parent-category-dropdown" href="' . esc_url( get_term_link($category) ) . '"' . '>' . $cat_name . '</a>';
        $indent = str_repeat("\t", $depth);
        $output .= "\t<li class='parent-category " . $cat_slug . "'>$link\n<div class='category-dropdown'>\n<span class='parent-category-title'>" . $cat_name . "</span>\n$indent<ul class='submenu'>\n";
    }

    // Configure the output for lower level list elements and their URL's
    if ( $depth > 0 ) {
        $link = '<a href="' . esc_url( get_term_link($category) ) . '"' . '>' . $cat_name . '</a>';
        $output .= "\t<li class='sub-category'>$link\n";
    }

}

// Configure the end of each element
function end_el(&$output, $page, $depth = 0, $args = array() ) {
    if ( $depth === 0 ) {
        $indent = str_repeat("\t", $depth);
        $output .= "$indent</ul>\n</div>\n";
    }
    if ( $depth > 0 ) {
        $output .= "</li>";
    }

}

}

Thanks,
James