You can create a SLA and attach it to the assignment. Then refer 3 date time properties in the SLA to set the goal,deadline and passed deadline. Now before coming to the assignment you just need to set those 3 property values.
one can always expose the SLA rule to Business Admin Access groups and maintained in a seperate portal
or Create the SLA rule with option by passing the Goal and Deadline from property. Write an activity where the property set can be done and in that property set create a "Dynamic System Settings" Property and get the value from the same. this DSS can be changed in production without any deployment.