When call load is uniform during the day, you can use Erlang to calculate staffing per hour, which will be the same throughout the day. If call load fluctuates, such as a different call volume during early morning, around noontime, etc., then you need to calculate staffing for each of these periods separately, and roster the shifts accordingly.
As for the time you lose for breaks, vacation, etc., which we refer to as shrinkage, you need to increase the headcount proportionally. Some Erlang calculators (e.g.
http://www.diagnosticstrategies.com/EasyErlang.htm) calculate shrinkage adjustments for you.
For manual calculation, the simplest approach is to increase headcount by the percentage of time lost to shrinkage, e.g. multiply headcount by 1.2 for 20% shrinkage. Bearing in mind that the additional 20% will also be subject to the same shrinkage, a safer approach is to multiply headcount by the shrinkage squared, e.g. 1.44.
See more about shrinkage calculations in
http://www.diagnosticstrategies.com/....htm#Shrinkage