A work formce managment center could help you out with these issues, i remember that we used to have that same problem here.
We started with the volume of the calls, if you don't have a big volume of calls at one hour you don't need that many agents, by using the method you just sayd, you can work something out ( need and demand) also you can calculate by using this calculator how many agents you will need at each hour.
http://www.erlang.com/calculator/call/
also this site could help you a lot, it sure helped me out, before i had BP.
Regards