For purposes of applying paragraph (4), with respect to monthly benefits payable for any month after December 1977 to an individual who was entitled to a monthly benefit as reduced under paragraph (1) or (3) prior to January 1978, the amount of reduction in such benefit for the first month for which such benefit is increased by reason of an increase in the primary insurance amount of the individual on whose wages and self-employment income such benefit is based and for all subsequent months (and similarly for all subsequent increases) shall be increased by a percentage equal to the percentage increase in such primary insurance amount (such increase being made in accordance with the provisions of paragraph (8)). In the case of an individual whose reduced benefit under this section is increased as a result of the use of an adjusted reduction period (in accordance with paragraphs (1) and (3) of this subsection), then for the first month for which such increase is effective, and for all subsequent months, the amount of such reduction (after the application of the previous sentence, if applicable) shall be determined—
(A) in the case of old-age, wife’s, and husband’s insurance benefits, by multiplying such amount by the ratio of (i) the number of months in the adjusted reduction period to (ii) the number of months in the reduction period,
(B) in the case of widow’s and widower’s insurance benefits for the month in which such individual attains age 62, by multiplying such amount by the ratio of (i) the number of months in the reduction period beginning with age 62 multiplied by 19 ⁄ 40 of 1 percent, plus the number of months in the adjusted reduction period prior to age 62 multiplied by 19 ⁄ 40 of 1 percent to (ii) the number of months in the reduction period multiplied by 19 ⁄ 40 of 1 percent, and
(C) in the case of widow’s and widower’s insurance benefits for the month in which such individual attains retirement age (as defined in section 416( l ) of this title), by multiplying such amount by the ratio of (i) the number of months in the adjusted reduction period multiplied by 19 ⁄ 40 of 1 percent to (ii) the number of months in the reduction period beginning with age 62 multiplied by 19 ⁄ 40 of 1 percent, plus the number of months in the adjusted reduction period prior to age 62 multiplied by 19 ⁄ 40 of 1 percent,
such determination being made in accordance with the provisions of paragraph (8).