Blending work makes calculating staff required that much more difficult.
I would suggest that to get close to the figure that you need that you look at each aspect in isolation. Outbound work and processing work can use a similar calculation although for outbound you will need to factor in time lost due to answerphone or unobtainable tones if the user is dialling without a dialler.
You should consider that you do not want to burn your staff out. no one can operate at 100% productivity. agents need around 15% of each time period waiting for calls over the longer time period to avoid burnout. Dont make the mistake of trying to squeeze this 15% as you may end up with a culture and staff turnover that you may not want.
you are in control of when you do admin and outbound calls but you are not when dealing with inbound.
How about calculating the number of outbound and admin items, multiply by the expected process time and then divide by say 25 minutes to give your handlers 5 mins break each half hour. The end result is staff you need.
Deal with inbound totally separately if you can.