How to take care of IGST levied on custom duty on Imported inputs

Hi all

Can anybody guide me that how to take care of IGST levied on custom duty on Imported inputs in Navision 2009. Do we have to post the custom duty through item charge (for applying GST) or is there any other way. Also custom duty rate varies from the normal exchange rate. So, how to take care of that

Thanx in advance,

Rahul Bhandari