The first thing to define is what does “understaffed” mean to you? Not meeting your service level expectations? Constant calls in queue? Or are you looking at long term planning and wanting to complete just in time training to keep up with your attrition?
In terms of meeting your service level objectives:
You need to determine your intra-day distribution of calls and handle time and compare it to available staff (butts in seats). A Workforce Management tool can greatly assist in this area. Depending on the size of your center, and your call volumes, this can fluctuate greatly from interval to interval (15 or 30 minute increments are typically used for staff requirements).
Let me know if this helps, or if I can provide you any additional assistance. You can email me: firstname.lastname@example.org