Crystal reports - Show a running subtraction and highlight at zero

Question

I have a report that shows a "Current Stock" value at Group Header. Within the group there are records that show how much stock is required per line item. I want to show a running total of how much stock will be left after each line item (these are sorted by date)

I would like it to present similar to this :

Group Item 1 - Stock : 5000

Group 1 Item Name	Demand	Remaining
First	3000	2000
Second	1500	500
Third	1000	-500

Thank you