Interest calculation based on Net Due Date

Hi guys: I want the interest calculation based on net due date and not document date or posting date.

I tried to check the box "ALWAYS CALCULATE INT FROM NET DUE DATE in prepare item interest calculation step, but system gives me message

"Do not select "Calc.intrst from net due date" for this int.calc.type

Message no. F7128"

Here are my questions:

1- how can i make the system to calculate interest only after the item is due for payment - right now its calculating anyday after the document date

2- Is this the correct place in configuration to do this? (ALWAYS CALCULATE INT FROM NET DUE DATE )

3- Also in T code FINT, how can i run interest posting for only one item rather than all that show up in test run? I tried selecting/highlighting only one but it posts all.

thanks much

brian