Calculate sum of under estimates

I have a 2008 r2 cube with 2 measures EstmatedTime and ActualTime and a date dimension.  I need to calculate the sum of the under estimates.  So if my estimated hours on the 1st, 2nd and 3rd are 10, 10, 10  and my actual hours are 11, 12, 7, I need to return 3 as the sum of the under estimates for the period.  Is there a way to do this using MDX, or do I need to do it in the ETL?

---

John Schroeder