(b)Tax shall be calculated on the total income of the relevant previous year as reduced by the additional salary or additional family pension, as the case may be, as if the total income so reduced were the total income of the assessee, and the amount by which the tax so calculated falls short of the tax on the total income before such reduction shall, for the purposes of clause (a), be taken to be the tax on the additional salary or additional family pension, under this clause.