How do I add a pivot table calculation that will show me the difference
between value fields over a particular column label?

In this case, the column label is SOURCE, which has 2 fields, platform and
ePEN. I'm trying to get the difference between platform and ePEN for various
values, something like:

SOURCE
PLATFORM EPEN
ORDER_COUNT ORDER_COUNT DIFF
SITE_ID 1131 10 9
1
255 35 40
-5