30.—(1) The base liability for a defined class of hereditaments for a relevant year beginning after the first relevant year of the relevant period (the year concerned) shall be found by applying the formula (BL × AF).
(2) For the purposes of this regulation–
BL is the base liability for the class of hereditaments for the relevant year immediately preceding the year concerned, and
AF is the appropriate fraction for the class of hereditaments for the relevant year immediately preceding the year concerned.