i am really bad at maths but i think this should be work
$Vote_up = 555;
$Vote_down = 555;
$total = $Vote_up + $Vote_down;
if( $total <= 0 ){
$score = -11;
} else {
$score = (($Vote_up / $total) + ($Vote_down / $total) * 10) * -1;
}
echo floor( $score + 11 ) . '/10';
if someone have a better solution, please i would like to know it - thanks!