need some help in subsummary report

Hi All,

I am new to FileMaker and I am trying to create a subsummary report for an existing database. I have to print invoice entries group by customer, object class. A customer can have many object classes. So the report should look like

Customer 1

Object Class1

invoice records

Object Class2

invoice records

Customer2

Object Class1 (for this customer)

and so on...

Now I have to compute total amount of purchase for every object class type for a customer and then apply some discount on them. For the total amount and discount, I have created two summary fields. Now I should display the net amount ( total amount - discount). I dont know how to implement this. Should I create a new field to store the difference between the two summary fields. In that case, what will be this field type. If it is not a summary field type, then can I place it in the sub-summary part of the layout.

Any help is greatly appreciated.

Thanks.