Mr.Bhaskar,
on the said subject, Sub Rule 3(A) (iii), Rule 6 of Cenvat Credit Rules 2004 will apply. this is specificaly for for common input cenvat credit bifurcation and bifurcation formula is as follow:
"The amount attributable to input services used in or in relation to manufacture of exempted goods and their clearanceup to the place of removal or provision of expempted services = (E/F) multiplied by G, where E denotes total value of exempted services provided plus total value of exempted goods manufactured and removed during the preceeding financial year, F denotes total value of taxable and exempted services provided, and total value of dutiable and exempted goods manufactured & removed, during the preceeding financial year, and G denotes total cenvat credit taken on input service during the month."
So bifurcate your total cenvat credit input accordingly.