The reference mark ( I 'M ) of the maximum luminous intensity , referred to in paragraph 6.3.3.2. above , shall be obtained by the ratio : I′M = IM/4300 This value shall be rounded off to the value 7.5 - 10 - 12.5 - 17.5 - 20 - 25 - 27.5 - 30 - 37.5 - 40 - 45 - 50 . ''