How do I calculate the values from individual fields?

How do I calculate the values from individual fields?

Hello, 

I am looking for a way to calculate the values from individual fields in the module "Invoices". 
  1. "Sub_Total" (it's the "offical" field of the Products subform)
  2. "Discount_in_Percent" (picklist)
  3. Result: "Discount_Value" (input field)

The result should be a value in USD. So the calculation should be: 
USD 130.00 (field "Sub_Total") * 30 (picklist "Discount_in_Percent") / 100 = Result: USD 39 (field "Discount value")

How can I achieve this? 

Thanks for help, 
M.