23Where section 23(4)(a) applies to exclude a gain which, in consequence of this Schedule, is not all chargeable gain, the amount of the reduction to be made under section 23(4)(b) shall be the amount of the chargeable gain and not the whole amount of the gain; and in section 23(5)(b) for the reference to the amount by which the gain is reduced under section 23(5)(a) there shall be substituted a reference to the amount by which the chargeable gain is proportionately reduced under section 23(5)(a).