Quote Discount/Surcharge should be calculated on Selling price/Total price instead of list price

Dear Guru's

As per the requirement, Quotation discount/surcharge should calculate on quote selling amount not on the list amount.But as per the standard functionality quote discount is calculating on the list amount and it is adding to selling amount.

Please find the below required example:

eg :  Quote  List Amount 10000

        Quote  Selling Amount 5000 ( Override the quote selling amount)

Given discount@ 10% through price adjustments window, system has to calculate 10% on 5000(Selling Amount) as 500 and deduct from selling amount(5000-500) and display quote total as 4500/-

Please help me on this..

Thanks,

Santosh.