S.C. Code Ann. § 12-21-6580

If a major tourism or recreation facility or a major tourism and recreation area is expanded or improved with an additional twenty million dollar investment being made within an investment period, the amount of admissions tax revenues subject to Sections 12-21-6530 and 12-21-6540 for the benefit period is the amount in excess of the annual admissions tax revenues previously generated by the major tourism or recreation facility, or by all of the tourism or recreation facilities within the major tourism or recreation area, as appropriate. This amount is determined by using the average of the admissions tax revenues generated during the twenty-four months preceding the date of the filing of the certification application.