Subject to section 7 of this Act and to subsection (2) of this section, where an application for postponement under section 3 of this Act has been granted for any rating period in respect of any farm land, payment of the general rate made and levied by the Council for that period in respect of that land shall be deemed to be postponed to the extent of such percentage as the Council from time to time by special order determines:

“Provided that payment of the general rate shall not be deemed to be postponed under this section beyond the extent at which the amount of the general rate the payment of which is not postponed is equal to the amount of the general rate that would be payable if the rateable value of that land were $18,000.”