Need formula & function for the question below

Neeraj Chavan

If I apply for PTO, I want to make sure it's getting subtracted from the "WR Balance Before Application" and it would be straight subtraction selecting cells & it's sorted. The main formula I am looking for is, If I apply again, my "WR Balance Before Application" should be the latest "WR Balance Left After Application" from my last entry. How can I do that? Immediate help would be a great help and appreciated.