Help Desk Measurements

I am in charge of a 9 peson Help Desk for a small company, that supports an Accounting Software package. Our users skillsets vary widely. Some are CPAs, others know nothing about accounting. Some are technical, some don’t even know what a floppy drive is. The software is also quite complex. Due to the above, and the impact these things have on call times. I need to devise some performance measurements for my staff other than the number of calls taken. However most of the metrics I read about require someone monitoring the call to see if certain objectives are met, and I am too short staffed for much call monitoring. Any suggestions or resources that you may know of would be GREATLY appreciated.