you can’t use any erlang-formula.
outbound projekts are more fluctuating, many factors will have an influence in this business, much more as in inbound projekts.
if you have a significant number of agents and contacts (min. 10 to 10000), you should get a predictive dialing system to get the maximum of call-performance. after then you are able to define 80 percent off all possible factors to calculate a right workforce/forecast.