The erlang calculations do apply for data calls too. However the calculation would not be directly linked to the data rate as you have asked for. For the planning purposes you generally covert the data usage into equivalent erlangs for calucalting the vocoder type of resources. There could be two approaches. One purely on the data side where you need to know the queing delay etc and apply Erlang C table. The other approach could be to convert data into equivalent voice erlangs. In this approach you will have to know upfront that the resources that you are planning for this, Is it affected by the data rate or know. For example to serve 153.6 kbps some system may require two resources where as some may only require one irrespective of data rate. In case of later, your question will yeild you 0.5 erlang.