Defaulting Promise Date to First Scheduled Ship Date

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We have a requirement to default the Promise Date to the First Scheduled Ship Date.  The first problem is that we can't do this based on a Post Transformation Rule linking Promise Date to Scheduled Ship Date because if the Scheduled Ship date is updated, then Promise Date updates as well. 

We're thinking that we need to default an EFF with the value of the Scheduled Ship Date IF the EFF is NULL, otherwise, no update.  Then we'd  have a second Post Transformation Rule to default the EFF contents to the Promise Date.  

The Second Problem is that Oracle's note (Doc ID 2305643.1)    defaulting the Promise Date is incomplete when it comes to defining the Post Transformation Rule to accomplish this and doesn't explain the details in arriving at the Value to set in Promise Date.