Create custom row in pivot table -- just need simple subtotal forumla

I have a pivot table attached below:

I just want to calculate the Gross Revenue which doesn't include credits/chargebacks...how do I set that up with this pivot table???

The formula is just sum of (authorize total - creditted) + paypal total - meritus chargebacks + credits.

This doesn't seem like it should be difficult, but it's driving me crazy....any help is much appreciated, thanks!!