Three columns used are Duration(dependent), Projected Duration(I initially copy all of the durations over when setting the project) and +/- days(calculates the difference between Duration and Projected Duration) The trick here is you need to strip the "d" from the value to allow computations.
=(VALUE(LEFT([Projected Duration]9, (LEN([Projected Duration]9) - 1)))) - (VALUE(LEFT(Duration9, (LEN(Duration9)))))