Service tax- ratio for taxable & exempt service

Can anyone suggest how to calculate ratio for taxable & exempt services?

Its a freight forwarding company wherein it undertakes inland transportation as well.  Service Tax is not charged on Ocean Freight (which includes profit margin)

Also if we show separate set of books are maintained for taxable & exempt services, then 100% input credit can be availed for input services directly used for dutiable services. Then what about input credit used for comman utility?? (Audit,Rent,telephone etc). Whether proportionate input credit can be availed as per the ratio calculated?

Please suggest..