protected function _getCustomTemplateFilename() {
        $filePath = $this->_formatTemplate(
            Mage::getStoreConfig('turpentine_varnish/servers/custom_vcl_template'),
            array('root_dir' => Mage::getBaseDir())
        );
        if (is_file($filePath)) { return $filePath; } else { return null; }
    }