Task duration related to Start Date

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I am setting up tasks in my process and want to make certain task due dates = to start date.  I realize the Duration field on the task is what determines the Due Date, but I have not figured out a way to base this off of the start date. 

I have thought about setting conditions to kick the task of 3 weeks before start date and setting the Duration to 15 days thus the Start Date, but if the process is not kicked off until 2 weeks prior to start date these tasks would be triggerd, but with a Due Date 1 week after start date.