4. In respect of any lump sum or instalment of a lump sum which became payable before 9th April 1990 but on or after 10th April 1989 there may be paid an increase of7.6 per cent of the amount of the lump sum or instalment (as increased by the amount under section 1 of the 1971 Act or under the existing Orders) multiplied by AB
(a)A is the number of complete months in the period between the beginning date for the lump sum or, if later, 10th April 1989 and the date on which it became payable; and
(b)B is 12.