law: General Clauses Act, 1897
  12. Duty to be taken pro rata in enactments
  Where, by any enactment now in force or hereafter to be in
  
  force, any duty of customs or excise, or excise, or in the nature thereof, is  leviable on any given quantity, by weight, measure or value
  
  of any goods or merchandise, then a like duty is leviable 
  
  according to the same rate on any greater or less quantity.