Calculate and Update child values when master value is changed

Hi

I have two tables 1-n

Master Table

Shipment (Shipment_No,Amount)

Child Table

Shipment_Details(Shipment_Line, Shipment_No, Unit_Price, QTY, FOB)

Now FOB is calculated as

FOB=(Unit_Price*QTY) - Amount
Now if i change Amount in Shipment Table how can i update calculation in child table can I use DAL and guidance.