I think you got some great answers on your original post. I think slambert said it best and let me try to put this in practical term for you. Every ACD reporting package will let you see the % of customers abandoning by seconds. By analyzing this data, you will quickly see how long your customers will wait until they hang up. It is customer dependent and industry dependent, but most of the time, majority of customers will abandon within 30 seconds to 60 seconds. If you can identify an ASA that will get to most of these customers, then that should be your target ASA and abandon %.
Additionally, I would do semi annual customer satisfaction survey of your customer base. They are the best source of information for you to see if your SLAs and your quality delivery are set correctly. If you are answering your customer calls timely and you are providing the right customer service to your base, then the survey will reflect that fact. If not, you will get that fact too from your customers. Workforce planning, service level management and quality customer service are high wire acts that we must balance correctly to satisfy both the external customers and the internal customers. If you set and manage your customer expectations properly, you can run a low cost and high efficiency call center operation that doesn't have to be a 100% cost center for your company.