We could not compute footer of a percentage column from it alone without knowing other column values. You have to divide footer values of other columns like this:

    Table::create(array(
        ...
        "showFooter"=>"bottom",
        "columns"=>array(
            ...
            "Conversion(%)"=>array(
                ...
                "footer"=>function($datastore)
                {
                    // Get total values of related columns and compute the total percentage yourself
                    $totalColumn1 = $datastore->sum("column1");
                    $totalColumn2 = $datastore->sum("column2");
                    $totalPercentage = $totalColumn1 / $totalColumn2 * 100;
                    return $totalPercentage . " %";
                }
            ),
        ),
        ...
    ));