Unfortunately this basic approach is incorrect. Mathematically speaking, you could argue that 30% shrinkage is not adjusted for correctly when multiplying the available time by 70% but rather by dividing it, but these differences are not likely to have pragmatic differences.
But more importantly, the assumption that in order to figure out headcount you simply divide the call load (call volume * AHT) by the call clearing rate is incorrect. The reason for that is that calls do not arrive in an orderly fashion, such that as soon as the agent finished one call the next caller arrives. The reality is that sometimes several calls arrive at once, or there are very few calls, and other times there are no calls. Said another way, AHT of 300 seconds does not mean a call every 300 seconds. In fact, most of the time calls will arrive much faster than that, and an interesting point is that with AHT of 300 seconds, the higher probability is to get calls slightly faster than once every 300 seconds.
Just as an illustration, the number of agents needed to support the call load and shrinkage, and service level of 80/20 in the example is actually 16
Please read the article
http://www.diagnosticstrategies.com/...c_modeling.htm for a detailed description of the background and the correct way to calculate staffing.