In sub-paragraph (b) of paragraph (i) of Rule 2 of the General Rules (which provides, in the case of a clergyman or minister of any religious denomination, for the deduction from any profits, fees or emoluments of his profession or vocation of a part, not exceeding one-eighth, of the rent or annual value of a dwelling-house any part of which is used mainly and substantially for the purposes of his duty as a clergyman or minister) for the words " one-eighth " there shall be substituted the words " one-quarter. "