I don't know of any offhand.
I do remember that Cool is a function of, among other things, time in combat divided by 10. If you keep that standard Cool should go down by a point every year or so.
You could allow a check every, say, 10 firefights? If you roll under your current Cool on a d10 you take a point off. This would make progress slow and slower yet the more experienced you get.
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