If you search on “erlang b” using a search engine, such as http://www.google.com, you will get many hits. A lot of these include free online calculators and a few give you the information for making your own. There are even a couple of Excel worksheets or add-ins available. The articles are worth reading, and the advantages and differences between Erlang B, Erlang C, Poisson Tables and other approaches ares spelled out. I suggest some time be spent in this area, as you are then more likely to get answers to your specific questions.