OK. This is a great idea, but I think that another solution could be much simpler:
No raise in staff fees after you sign them until the end of the season!
The staff fee would be the same every week for the whole season, and it would be much easier for a manager to keep track of how much he's spending.
On ZyZla's 24 week suggestion, this is much too long. Until the end of the season or 12 weeks would be a better time period.