How to create a Calculated Field based off two Avg fields in a Pivot table?

Hello,
I am having trouble to do the following.
I have a Pivot table that pulls the Avg of two fields for two months, see example below.

Avg Gross $ Avg Net $
Jan 2014 20 10
Feb 2014 30 20


sample 1.png
See sample attached.

The Avg Gross and Net is shown by going into the values and selecting "Summarized value by -> Average".

On the right side of this Pivot, what I wanted to do is to show a Avg Gross to Net $ in this pivot. So the formula should take "Avg Gross $" - "Avg Net $" = Avg Gross to Net $.

I am having trouble calculting this new field in the pivot table using a calculated field because the Calculated field pulls the variables from the existing field list and there isn't a field called "Avg Gross/Net"....I need to find a way to calculate the Avg Gross to Net into the Pivot table so I can pull a pivot graph out of it.

I hope this makes sense. I hope someone can help!!!

Thank you!!