Need Pivot Table assistance

I have two columns in a pivot table that are % of column calculations and I need to subtract these two columns. I can't do it using calculated fields under formulas because the columns are not fields - there are % of column values of two different fields. Thoughts on a solution to subtracting the columns? I have the data below as an example and simply need a way to get the difference of the columns within the pivot table

Sum of 2010 NEP Sum of 2011 NEP
8.38% 8.42%
20.71% 21.34%
24.77% 24.95%
21.70% 21.64%
24.44% 23.66%
0.00% 0.00%
100.00% 100.00%