One more question in function

Dear Friends.

I am making a chart. It contains 3 subject
01. OT Forecast
02. Approved
03. Balance

* The Balance = OT Forecast - Approved ( this is my function)

* When I update the approved amount it will automatically deduct from balance...


Now I need a function for >
When the approved cell comes empty, the balance should show the "0"
but the total amount should be correct....

Sorry for my poor english ...

Irsaath