4Where the qualifying period of pensionable service performed by a scheme member consists of or includes part-time service performed after 1st January 1987, such proportion of the rate of pension which would have been payable if that service had been whole-time as the Board may determine, in accordance with general directions of the Commissioners, shall be payable in respect of that service; except that in calculating the length of the qualifying period of pensionable service no account shall be taken of any year or part of a year in which the amount received by the scheme member in respect of stipendiary ecclesiastical service is less than such amount as the Board, with the concurrence of the Commissioners, may from time to time determine.