Know-How Exchange : Research/Metrics

Does anyone have a method for analyzing brick and mortar retail door traffic? We have a door counter in place that measures breaks in a beam at our only door to the store.

Problem is that it's not as simple as cutting the number in half to determine traffic. Employees, mail man, UPS man, etc. are counted as well. Is there a universally accepted factor to apply to the gross count that will determine (with a small margin of error) customer traffic?

Thanks!