Coupon code Discount should apply for MRP instead of selling price

Question

Currently after applying coupon code, discount is applied to selling price. but we want Discount to be applied for MRP and not for selling price.

Example :

MRP = 200   , Selling Price = 175  
Coupon code discount =  10% 

so how Default magento works is = > 10% for 175 [final price = 157.5 ],

but what i need is 10 % for 200. [final price = 180 ]

so i tried below code as mentioned in this link,

now what i am getting : 175- 20 = 155 as final price

but What i need is : 200- 10 = 180 as final price

is there any Module to achieve this or what code i need to change to get result?

    $item=$observer['item'];
    $rule=$observer['rule'];
    $rule->getDiscountAmount();
    $rulePercent = max(0, 100-$rule->getDiscountAmount());
    echo "<br>--pro id---".$item->getProductId();
    //echo "<pre>";
    //print_r($rule->getData());

    if($rule->getSimpleAction()=='by_percent')
    { 
        $disPer=$rule->getDiscountAmount();
        $product=Mage::getModel('catalog/product')->load($item->getProductId());

        //echo "<br>pro price----".$product->getPrice(); 

        $item->getDiscountAmount();

        $ratdisc=($product->getPrice()*$disPer)/100;
        $DiscountAmount=$ratdisc+$item->getDiscountAmount();
        $item->setBaseDiscountAmount();
        $BaseDiscountAmount=$ratdisc+$item->setBaseDiscountAmount();

        $result = $observer['result'];
        $result->setDiscountAmount($DiscountAmount);
        $result->setBaseDiscountAmount($BaseDiscountAmount);
    }