Hi again Sam,
The calculations is based on Erlang C , a long-established statistical model for analysing systems involving queuing. The algorithms are not our own and a quick google search will turn them up. They make some simple assumptions about the traffic, details of which can be found at the CC-Excel FAQ.
Calculations are traditionally done the other way round. In other words “how many agents do I need if I want 80% of calls answered within a specified time. We reversed the process for the CCXLService() functions. We have a Windows calculator called Westbay Traffic Calculators that includes Erlang C calculations. Out of interest, I ran your numbers through it to confirm the results matched those from CC-Excel. I also changed the target time to twenty seconds to see the effect, which was to increase the estimated agents required from 21 to 23. Here are some screenshots:
All the best,
Tom H.