Allocate Stock against Monthly Sales - Reducing to zero

Hello,

I am trying to find a formula to allocate stock against monthly sales until it is then exhausted, and then move on to the next stock item. This will help calculate what months stock will be exhausted.

The total of each stock item is subject to change so I want to be able to insert a formula into the table that will do this automatically rather than going through and doing it manually using subtotals etc. in row 10.

For example. Stock item 20156 we fully supply Feb & Mar but only 3,038 of March sales of 8651 - So the balance then needs to then be allocated from the next stock item(20165)

Any help would be greatly appreciated.

Bob