Hi I need to remove 10% from a shopping carts subtotal

Original code:
Code:
<?php echo number_format($order->subtotal,2);?>&OID=<?php echo $order->trans_id;?>
I know it's not precise, but would something like this work?

Code:
<?php echo number_format($order->subtotal * 0.909090909,2);?>&OID=<?php echo $order->trans_id;?>
Thanks