This will have “seasonal cycles” obviously and should not be used to penalize departments. If a department has longer service employees because they do a great job of maintaining loyalty/treating employees fairly, then those employees will be eligible for more vacation probably. So this has to be balanced against length of service as well.
I created a “burn out ratio” which is the number of employees (percent) that are near or at max for their vacation balances.
Department ratio of work days (or hours) to utilized vacation days (or hours).

