Yo lo tengo implementado así:
Código PHP:
$total_ratings = $row['total_ratings'];
$total_rating = $row['total_rating'];
$current_rating = $row['rating'];
$new_total_rating = $total_rating + $rating;
$new_total_ratings = $total_ratings + 1;
$new_rating = $new_total_rating / $new_total_ratings;
mysql_query("UPDATE links SET total_rating = '".$new_total_rating."' WHERE id = '".$id."'") or die(mysql_error());
mysql_query("UPDATE links SET rating = '".$new_rating."' WHERE id = '".$id."'") or die(mysql_error());
mysql_query("UPDATE links SET total_ratings = '".$new_total_ratings."' WHERE id = '".$id."'") or die(mysql_error());
Donde $total_ratings es el numero de votos y $total_rating la calificación total.