Calculate number required depending on previous months' results

Hi
Hope someone can help?

I need to write a formula that will solve the following problem, but so far I'm not having much luck.

I work in a call centre and my job is to assess the staff's calls.

Each month they have 7 calls listened to. If they score above 96% they stay on 7 calls the next month. If the score <96% they need 12. If those 12 score below 96% they would need 17, but if they reach 96% they go back to 7 the following month. 17 is the maximum, so when on 17, they will either stay at 17 (if they score <96%) or drop to 12 and then to 7 etc.

Hope this makes sense? I want this calculation (I.e 'how many calls does John need listening to in September?) to be automated, depending on his scores for the previous months!

I might be wanting too much, but if anyone can suggest a way forward, I'd very much appreciate it.

Hope my explanation of the scenario is clear!

I've created a 'mock up' of what a three month 'summary' would look like, but it's hard to describe (hopefully it makes a little more sense).