In determining the voluntary contribution payable by a person in a contribution year in which he or she -

(a) becomes a voluntary contributor, or

(b) ceases to be a voluntary contributor -

(i) on becoming an employed contributor, or

(ii) in the case of a person born before 1 January 1958 on attaining pensionable age, or

(iii) in the case of a person born on or after 1 January 1958, by reason of having been awarded the State pension (contributory) or attaining the age of 70 years,

his or her reckonable income for the preceding contribution year shall be calculated as a share of his or her reckonable income in that year proportionate to the part of the contribution year in which he or she becomes or ceases to be a voluntary contributor.