Adding a calculated row in a Matix

I am trying to create a live and dynamic inventory tracker to undertand how any units I will have on hand by month.

I am not totally sure this is the best place to do it, but this is how I do it in Excel.

I want to create a calculated dimension or add calculated row in a Matrix table to give me a delta by month, and then a rolling delta.  The image is what I have so far. I need to add a caludation under each Style, that subtracts the demand from the on hand plus the open PO. Then do it in a rolling fashion.