How to change shipping rate in Percent

app/code/core/Mage/Shipping/Model/Carrier/Flatrate.php

find line

$method->setPrice($shippingPrice);

and put following code above that line

$session = Mage::getSingleton(‘checkout/session’);

$quote_id = $session->getQuoteId();

$item_quote = Mage::getModel(‘sales/quote’)->load($quote_id);

$shippingPrice = $item_quote->getSubtotal()*($this->getConfigData(‘price’)/100);