public function getFlex($size = null)
    {
        if (is_null($size)) {
            $size = $this->baseSize;
        }
        $getter = $size . 'Columns';
        $flex = $this->getMaxColumns($size) - $this->{$getter};
        if ($flex < 0) {
            return 0;
        }

        return $flex;
    }