How to calculate value based on DFF value selection on requested delivery date and in transit times

Summary:

Ex-factory request date is a DFF with date data type created at PO schedules. It should be auto-populated / auto calculated based on Requested Delivery Date and transit days. The logic required is Ex-factory requested date DFF should be auto calculated and populate the date based on the requested delivery date on PO and In -transit days ( Requested delivery date - In-transit days)
Currently in EBS, client has this logic implemented. 

Please suggest if any query needs to be implemented

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