Times the number of eigrp updates/hello's/routing table updates/numbers of routing table changes/eigrp ack's * 400 will give you the bw required for EIGRP only.

So in short - you just cannot work out how much bandwidth you will need for just EIGRP. You need to work out how much BW you are going to need during normal production hours for 1 site, then times by 400.