Transmisson capacity for EDAP can be calculated not from Erlang formula – because it is not CS traffic, but PS traffic, so you just need peak values of your traffic and prefered QoS parameters.
Example.
If you are expecting 2Mb packet traffic(EDGE/GPRS) from your BTS – you should dedicate 1xE1 for your EDAP.
Also – if you will have 2Mbit packet traffic at BTS- I can say you are happy man 🙂