Value of taxable service cannot include service tax portion. You need to reduce the ST component from the gross amount to arrive at the taxable service component.
Generally service Tax separately charged from the customers if you are not charging service Tax separately it means gross amount includes service Tax also.. You can segregate service Tax and taxable value and pay service Tax on taxable value
Valuation is to be done on the basis of gross amount charged for providing such service.ST is charged on the gross amount received and can not be part of the gross.