You could certainly measure efficiency based on calls per agent, if the quality of the answers they provide doesnt count
It is not uncommon for management, who doesnt understand the dynamics and the (often conflicting) parameters influencing the operation of the help desk, to focus on a single and easy to understand metric. You can find a white paper describing the theory of call center operation on
http://www.diagnosticstrategies.com/papers.htm which might help you educate management and agree on metrics that consider both the company and customers as well as the well being of the help desk staff.
When identifying metrics and performance criteria, I suggest that you use industry benchmarks very cautiously. Recent study we conducted shows that many often quoted industry metrics are not reliable enough to be applied without sufficient background as of how they were obtained . The results of this study are also published at
http://www.diagnosticstrategies.com/papers.htm.
I hope this will help you in your effort,
Joe