So I have a pivot table, here is a simplified version. Basically, data by category (and sub category) over time. What I want to do is a 3 step calculation process.
First step - what is the year on year growth rate of each category. Thats easy with the calculated field % change. Then I want the weight of each item by category and year. Again easy through the calculated field.

Now for the part I cant do. How do I now get the simple difference of year on year growth rates that I calculated earlier on. Then multiply those results by the weight.

So my pivot table should have the raw data, then the Year on Year growth rates, then the weights by year then the difference of those year on year growth rates, then the difference * weights. No matter what I treid, I cant create a calculated field that does a calculation on another calculated field.
I also tried creating a calculated Item that does all of those steps in one step but I also couldnt get that to work.
Oh and my real data set is 1.5 million cells, so i really dont want to do this offsheet - I want it done in the pivot.

Row Labels 2006 2007 2008 2009 2010 2011 2012
cleaning 38227 37122 31917 22751 17671 15978 48399
IT 33761 8193 32101 40127 44449 22716 27072
marketting 21135 13746 34257 45979 40028 20940 50748
Photocopies 42852 32500 15140 16290 31955 36128 19481
salaries 20154 34434 38315 20599 32908 34269 27220