Pivot Tables Find difference between 2 columns where fields are not in data set

[dji]

Hi
I have a data set where the data has a column called Fin Yr and the column is populated as FY15 or FY16
When I show these in a pivot table column view (across) I want to create a new column to show me the difference between the Fin Years
Can someone please advise how this is done as a calculated field where I cant reference the individual years .
Appreciate any help and example

Thank You