Dynamically setting event custom field value

I am using Event Tribe plugin. I wish to retrieve a custom variable associated with each event and decrease it by one. I have come up with the following I have not run it in case it causes problems! Does it look ok? Thanks for any help here

$holdvalue=$custom_fields['Places Left'] ;
$holdvalue = $holdvalue -1;
$args = array(
 'Places Left' => $holdvalue
);
tribe_update_event($event_id, $args);