http://www.engineeringtoolbox.com/ai...oom-d_867.html this is pretty good information on air change rates, but you also need to figure if your fresh air that you currently bring into the fireplace is big enough to support your chimney size.
You should also verify that the chimney is built correctly, chimneys are almost never built properly. Building inspectors don't know what they are looking at, and masons don't always know how to build them either.
We are trying to help you, we don't come on these forums just to bash people. Sometimes it come across that way, but try and remember that the forum doesn't convey tone very well.