Last bucket size limit

Hello, I have a following problem, hope somebody can help.
I have a total count (TC) of agents (hard limit - number of seats) and I need to display their statuses over time: AVAILABLE, BUSY, ON_BREAK and ABSENT. FIrst three statuses should be result of the term aggregation, but the ABSENT one should be TC - (size(AVAILABLE) +size (BUSY) + size (ON_BREAK)).

The deepest I was able to dig is to use threshold for the total count, but have no clue how to influence the size (or in other words cap it) to the TC - (sum of other bucket sizes).

Thanks,
Milan