Colins definition is the standard method to calculating utilization. I also agree that as utilization nears 80% you have to watch for signs of burnout and deterioration in service quality. The metrics that cover the other operational characteristics discussed in the other posts are:
* Shrinkage planned and unplanned unavailability due to vacation, training, sick time, etc.
* Schedule Adherence staffing shifts (usually at start/stop times) as planned
It is important to understand the cause and effect relationships between different metrics. You can control shrinkage and schedule adherence, and, consequently availability. However, utilization is the outcome - the ratio of call load to availability.
Additional resources:
http://www.diagnosticstrategies.com/...c_modeling.htm
http://www.diagnosticstrategies.com/multiQ.htm
http://www.diagnosticstrategies.com/service_level.htm
Joe