If, because of any change after the date of this lease in the method used to compile the index or for any other reason it becomes impossible or impracticable to calculate fairly the fraction referred to in paragraph 4.1.2 by reference to the index, or if any dispute or question arises between the parties to this lease with respect to a such calculation pursuant to paragraph 4.1.2 or with respect to the construction or effect of this provision, then such dispute or question shall be referred to an independent expert pursuant to paragraph 9 of this schedule.