Acheive a a declining subtraction to 0 rather than a negative number

Hi all,

I'm not sure how to explain my problem, so I've attached a sample spreadsheet. I think that might be clearer than my explanation.

What I'm trying to achieve is a declining subtraction to 0 rather than a negative number, as I've shown in row 25.
Also in row 22 I would like the values to show 0 from the left until the last positive figure in row 25.
In cell B21 I would like to show the figure carried over as shown in cell K22.


Many thanks