Import duties are always calculated in CIF value. If your buying is in CIF then your duty would be calculated based on your billing but if your billing is in FOB or CFR then the government will convert it into CIF billing & then duties would be calculated based on that.
Government formula for conversion of FOB = FOB + 20% Freight + 1.125% Insurance = CIF
Goverment formula for conversion of CFR = CFR + 1.125% Insurance = CIF
Goverment formula for conversion of CIF (no converson , direct calculate duty on it) = CIF
In short, the CIF value would be considered as an Assessable Value for the calculation of import duty.
Official Government Import Duty calculator is available in www.icegate.gov.in -> Custom Duty Calculator -> Trade Guide for Imports -> Enter products HSN or Description -> Get a calculator, enter Assessable Value CIF in it & you got an answer. -> Hit show bifurcation -> See IGST amount that would be refunded (adjusted as Input Tax Credit) when you sell it with the bill in a local market and collect GST in domestic sales)