Matrix with Variance Column

Hi folks,

I have a matrix with the follow format

I need to insert a new column to show the variance between this columns like bellow:

Im having some headeach to do it, someone have any idea to solve it?

(The variation is like: SUM(2012-2011)/SUM(2011+2012 values)

I'm have alread read the link http://www.simple-talk.com/sql/reporting-services/advanced-matrix-reporting-techniques/  this sample is similar but not equal my scenario and the differences turn the application of this sample invalid for my scenario.

Thanks to all!