Accounting for import purchase

I Had a query with reference to recording of import purchase. Whether the rate of exchange mentioned in the Bill of lading will be used as rate of exchange or the rates that are notified by RBI will be used . Also  will it make any difference if we had made a part payment in advance & remaining after delivery of goods. I also want to just know whethet thenon refundable duty will be included in purchase or not(as something CVD charged is claimed back). Please clarify.