“Taxable value” means the base year full value adjusted for any given lien date as required by law or the full cash value for the same date, whichever is less. In the case of real property which, prior to the date of the change in ownership or completion of new construction, was assessed by the board pursuant to Section 19 of Article XIII of the California Constitution, “taxable value” means that portion of the state-assessed value determined by the board to be properly allocable to the property which is subject to the supplemental assessment.