Getting an Advanced Custom Fields field_key from a field_name and field_group.

get_field_key

/**
 * Get a field key from a field name. Why isn't this an 
 * ACF API function?
 *
 * @param string $field_group The name of the field_group to search
 * @param string $field_name The name of the field to find a key for
 * @return string|bool The field key if found, else false
 * 
 * @author Simon Wheatley
 **/
protected function get_field_key( $field_group, $field_name ) {
        $field_definitions = apply_filters('acf/field_group/get_fields', array(), $field_group );
        foreach ( $field_definitions as $def )
                if ( $field_name == $def[ 'name' ] )
                        return $def[ 'key' ];
        return false;
}

Hi Simon --
Super handy ... if I could only make it work ;-)

What value does this function take for $field_group? Individual fields have "Field Label" and "Field Name", which I set in ACF's UI. But there's nothing similar for the Field Group.
In ACF, I have a group "My Group". Should I pass "My Group"? "my-group"? Some id value that I need to find in the DB?