Accrual offer on Net Order line amount

Hi

I've created an Accrual Budget and provide a discount of the applicable Category/Item. At the moment, discount is being calculated as a percentage of the gross value of the Order Line. So if the Order Line's Total Amount is 50 and discount percentage is 10 then 5 will be calculated as discount.

I would like the discount to be calculated on the net amount. So in the above example, if there is a price adjustment and the Line Total value changed from 50 to 40, then I would like the discount to be on 40 and not on 50. So the discount amount would be 4 rather than 5.