How to Define discount Percentage on Order and Ensure it should Apply on Shipment cost

Summary:

In Our Business Scenario we provide some discounts on the Agreed contracted price on adhoc basis and that Percentage of the discuount can be captured on the order so we want that percentage to be caluted as discount amount during the planning of the shipment.

So lets say.

We have created an Order #1 and mentioned the Percentage as 10% over order

Created a contract with 100 $

After the planning the final cost should be over the shipment is 900 and the cost details shoule be as below.

Base cost is : 1000 $

Discount is : -100(As per the percentage)

I need to know what RBI condition to be used to achieve this.