Subject to the provisions of sections 160 and 163, when rent is to be determined or commuted into fixed money rent, or the fixed money rent or the bighori determined under section 151 is to be abated or enhanced, the court shall calculate the rent--
         (a) in the case of hereditary tenants, in accordance with the rates sanctioned for hereditary tenants; and
         (b) in case of occupancy and exproprietary tenants, in accordance with rates which shall conform to the scale prescribed for such tenants in relation to hereditary tenants under the provisions of sections 63 and 64:
     Provided that, for special reasons to be recorded, the court may modify the sanctioned rates applicable to any particular case, and it shall modify such rates if it finds that, as a result of their application, the rent arrived at on commutation, abatement, enhancement or determination, as the case may be, is substantially different from the money value of the rent payable by the same class of tenants under the provisions of sections 63 and 64.