Magento partial refund programmatically - magento

Question

How can i create partial create memo? i.e. if total order value is £50 and I want to create a credit memo of £10. I am using the below code to create credit memo:
function createCreditMemo($orderId) {
$order = Mage::getModel('sales/order')->load($orderId, 'increment_id');
if (!$order->getId()) {
print_r('order_not_exists');
}
if (!$order->canCreditmemo()) {
Mage::log('cannot_create_creditmemo', null, 'product.txt');
print_r('cannot_create_creditmemo');
}
$data = array();
$refundToStoreCreditAmount = '10.22';
$service = Mage::getModel('sales/service_order', $order);
$creditmemo = $service->prepareCreditmemo($data);
// refund to Store Credit
if ($refundToStoreCreditAmount) {
// check if refund to Store Credit is available
if ($order->getCustomerIsGuest()) {
print_r('cannot_refund_to_storecredit');
}
$refundToStoreCreditAmount = max(
0,
min($creditmemo->getBaseCustomerBalanceReturnMax(), $refundToStoreCreditAmount)
);
if ($refundToStoreCreditAmount) {
$refundToStoreCreditAmount = $creditmemo->getStore()->roundPrice($refundToStoreCreditAmount);
$creditmemo->setBaseCustomerBalanceTotalRefunded($refundToStoreCreditAmount);
$refundToStoreCreditAmount = $creditmemo->getStore()->roundPrice(
$refundToStoreCreditAmount*$order->getStoreToOrderRate()
);
// this field can be used by customer balance observer
$creditmemo->setBsCustomerBalTotalRefunded($refundToStoreCreditAmount);
// setting flag to make actual refund to customer balance after credit memo save
$creditmemo->setCustomerBalanceRefundFlag(true);
print_r($refundToStoreCreditAmount.'<br/>');
//die('2');
}
}
//Mage::log($creditmemo, null, 'product.txt');
$creditmemo->setPaymentRefundDisallowed(true);
$creditmemo->register();
$orderCreditMemoStatusCode = Mage_Sales_Model_Order::STATE_CLOSED;
$orderCreditMemoStatusComment = 'Order Refunded.';
$saveTransaction = Mage::getModel('core/resource_transaction')->addObject ($creditmemo )->addObject ( $order )->save ();
$order->addStatusToHistory ( $orderCreditMemoStatusCode, $orderCreditMemoStatusComment, true );
$notifyCustomer = true;
$comment = 'testing refund';
$includeComment = true;
$creditmemo->setEmailSent(true);
// add comment to creditmemo
if (!empty($comment)) {
$creditmemo->addComment($comment, $notifyCustomer);
}
try {
Mage::getModel('core/resource_transaction')
->addObject($creditmemo)
->addObject($order)
->save();
// send email notification
$creditmemo->sendEmail($notifyCustomer, ($includeComment ? $comment : ''));
} catch (Mage_Core_Exception $e) {
print_r('data_invalid', $e->getMessage());
}
echo $creditmemo->getIncrementId();
}
In the below code, even if I set the $refundToStoreCreditAmount = 10; it still refunds the full amount. Any idea how to refund partially?