Pivot Table Calculated Field Showing Differences of Subtotals

Ok, I am missing something simple here or something. I am pretty new to Pivot Tables but they seem to make a lot of sense to me. I have a table pivoted that includes sales data for two fiscal years. I want to compare month to month from these two years for year to date. I get that no problem. I show the five months of our fiscal year and 2015 and 2016 Subtotals perfectly. But I want to add two additional columns that show the difference in the 2015 and 2016 FY Subtotals and then the percentage change between the two years. I can easily add the columns and do the math outside the pivot but that is not what I want because I am doing other things with it. I have tried every combination I can think of and it is just not working. Any help would be appreciated.

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