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As far as we know you have two options to fill DFF one is to assign it to LOV to pick a value not to enter, the second one is a free text to let the user enters any values he wants.
Please let me know, if anyone is not clear on requirement, I will explain more in details.
Your default will be evaluated too early. You should try to use a personalization to achieve this.
Form Personalization will be best option for you on this.
You can create Trigger on New Block (then Object is CONTROL_DIST) or trigger on New Item (then item in lines but after the line amount field) and then in Actions, probably the Trigger Object will be LINE_SUM_FOLDER.LINE_DF (as you said it is DFF)