Regarding Calculation of EC and SHEC wrt import of goods.

on many websites it is shown that EC and SHEC are calculated on BCD but some websites are showing calculation of EC & SHEC on value of BCD+IGST. does anyone know the proper method to calculate EC and SHEC w.r.t import of goods and also if there is any real invoice or clarification issued by CBEC, kindly provide that too. thanks