Help with IF-function

Hi!

I'm trying to use my Excel-document for staffing at a Call center. I have all the data I need, but I want column F (Agents) to automatically find a number that will give corresponding row in coumn F (Service level), let's say 80%.

How do I manage that? Quite new to Excel I'm afraid

Attached a sample file as well. It's the leaf that's called "staffing" I'm referring to. Hope I was clear enough in my explanation