S.C. Code § 12-6-3500
If the right to receive retirement income by a taxpayer allowed the deduction pursuant to Section 12-6-1170 was earned by the taxpayer while residing in another state which imposed state income tax on the employee's contributions, a credit is allowed against the taxpayer's South Carolina income tax liability in an amount sufficient to offset the taxes paid the other state. This credit must be claimed over the taxpayer's lifetime. The department shall prescribe the amount of the annual credit based on the taxpayer's life expectancy at the time the taxpayer first claims the retirement income deduction pursuant to Section 12-6-1170, and may require the documentation it determines necessary to verify the amount of income tax paid the other state on the contributions. Regardless of the tax rates applicable on the contributions in the other state, the total of the credit allowed may not exceed an amount determined by multiplying the contributions taxed in each year by the marginal South Carolina individual income tax rate for that year.
S.C. Code § 12-6-3500