Sorry I should have explained a bit more, it's what you mentioned in post #4.
So the user wants to be able to call up the customer details form, click into the discount sub form and see a list of all the products and the corresponding discount for those each of those products, for example below;
Customer A (Form) Discount (Subform) Product Standard Price List Price Discount % Discount Price Product 1 £5.00 £20.00 8 £18.40 Product 2 £2.00 £10.00 10 £9.00 Product 3 £20.00 £50.00 25 £37.50
Does that make more sense? then the user can also go in and update the discount percentages which will then intern update the discount price field.