In my opinion, your target for abandonment should be zero. You should not be planning for any calls in to your call center to be dropped before they reach an operator.
The Erlang C traffic model assumes that no calls are abandoned – that is, all calls remain in the queue until they are answered. If that is not the case, then you should be looking for ways to make it so, which usually means making more staff available for answering the calls. We have a technical paper that describes how to estimate how many agents your call center needs, with examples.
We also offer a free online Erlang C calculator for call centers. As you can see, abandonment is not a factor in the calculation.