180. For the purposes of section 179, the total pensionable earnings of a contributor attributable to contributions made under this Act are an amount equal to the amount that his total pensionable earnings would be if the unadjusted pensionable earnings of the contributor for each year were that proportion of such earnings that(a) his earnings on which a contribution has been made under this Act, calculated as provided in subparagraph 1 of paragraph b of section 98,
are of
(b) the aggregate of his earnings on which a contribution has been made under this Act and under a similar plan, calculated as provided in subparagraphs 1 and 2 of the said paragraph.