I would calculate this using the formula:
ServiceTime = QTime(1-((1-Confidence)/ErlangC))*3600
Where Servicetime is the number of seconds in which the required percentage will be answered. QTime is the average time queueing for those calls which queue (i.e. excludes calls which go straight through to an agent). Confidence is the percentage for the servicetime. ErlangC is the percentage likelyhood of the call being queued.
QTime can be calculated by:
QTime=1/((Agents * 3600/AHT) – CallsperHour)
Where Agents=number of agents available. AHT=Average handle time in seconds. Callsperhour=Calls per hour!