S.C. Code Ann. § 12-6-2290: Apportionment of remaining net income from principal profits or income derived from sources not otherwise described by this chapter.

If the principal profits or income of a taxpayer are derived from sources other than those described in Section 12-6-2252 or Section 12-6-2310, the taxpayer shall apportion its remaining net income using a fraction in which the numerator is gross receipts from within this State during the taxable year and the denominator is total gross receipts from everywhere during the taxable year. For purposes of this section, items included in gross receipts are as provided in Section 12-6-2295.