Dimensioning the IP Bandwidth ??

I am Dimensioning a GSM (R99) network; currently the network is 2G GSM, now i want to migrate the bearer from TDM to IP. After the Migration, only the radio side will remain the same as 2G and the core side will be on 3G (MGW and MSC-Servers will be introduced in the network). The problem is that like in 2G the TLF (trunk loading factor for voice) is normally 0.8 i.e. 80% for TDM (E1). For my voice traffic there is a media gateway after BSC, I want to send this Voice traffic on the IP as bearer, the total bandwidth (out going and incoming traffic) is 20Mbps for example from MGW. After MGW there is Access router(co-located with MGW) which is connected to CORE router located in other Major city, how much Light load Factor should I consider? Should I provide the total bandwidth between two router 30Mbps, 40 Mbps etc….?? Or what is the calculation for the Loading factor? Not Compromising the QoS for Voice.Please guide me?.