Busy Hour Factor

So I’m having an issue in my call center. We are using the Westbay calculator for staffing and the busy hour factor has a huge impact to that. The question is how is this accurately calculated?

The help says that 17% is a reasonable figure for a business which operates an 8 hour working day, but a higher figure could be entered if the business in question operates a shorter working day, or if frequent calls are being made to a different time zone.

So what about a 15 hour day or 18 or 24? Is it safe to assume that a queue with a longer hours of operation would have a smaller % busy hour factor? Right now based on the calculation I was given of PEAK HOUR CALLS x AHT (min) / TOTAL CALLS OFFERED x AHT (min)is coming up with a factor of 9.7% in a 15 hour call center but this does not jive at all with what our headcount requirements should be with our current performance.

Please help