Derive Function using Change Request Priority Code

Hi Experts:

I have requirement to Create a DERIVE function:    Derive  "Due Date"  on Customer Change Request based on "CR Priority" Code.

This would be a Create Customer CR.  7.0

Example:

    IF  Priority Code = 1 (Urgent)   then>  Due Date = Current Date + 1 Day (work day)

    IF  Priority Code = 2 (Standard)   then>  Due Date = Current Date = 2 Day (work day)

How do I get the Priority Code and Due Date available in the rule-set.  What Entity do I use to create the DERIVE Function? 

   any suggestions is greatly appreciated.

Thanks for your assistance.