How to calculate when work can be done

I have in the workbook a forecast of volume and based on some other metrics what the clearance capacity is. (column J & K) then in Column L I have calculated what the workload is that cant be completed on the day and will need to be sorted at the next available period. I have in column O&P put in what the formula needs to return. what I cant work out, is how to automate this, because I need to need it for multiple areas and also in the room change what the forecast will look like.

any help would be great.

The following formula yields the expected results for cells O3:O23:

Formula:

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Note that the formula is placed in column Q so that it can be compared to the expected results in column O.
The formula deviates from the values in cells O24 and below.
Cell M24 indicates that there is no spare capacity yet O24 displays 50. Please explain how the values in O24:O26 are calculated.

For the post carry over column try:

Formula:

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