(b)Where a registered dealer has not been assessed to tax for any previous year or part thereof but has furnished the return as required by sub-section (1) of Section 18 for any period, the tax payable in advance shall be an amount which bears to the maximum amount of tax payable according to any such return the same proportion as the period for which the tax payable in advance bears to the period for which such maximum amount of tax was payable according to such return.