payment calculation base

(i) in connection with a title II disability beneficiary, the average disability insurance benefit payable under section 423 of this title for all beneficiaries for months during the preceding calendar year; and

(ii) in connection with a title XVI disability beneficiary (who is not concurrently a title II disability beneficiary), the average payment of supplemental security income benefits based on disability payable under subchapter XVI (excluding State supplementation) for months during the preceding calendar year to all beneficiaries who have attained 18 years of age but have not attained 65 years of age.