The percentage shall be one hundred if the specified minerals do not extend beyond the boundary of the protected works, and shall diminish by one for every one-and-a-half yards by which the specified minerals extend beyond that boundary on each side thereof.

If the specified minerals extend to one hundred and fifty yards or more beyond such boundary, no payment shall be due by the company.

If the specified minerals extend further from such boundary on one side of the railway than on the other, they shall, for the purposes of this schedule, be deemed to extend beyond such boundary for the mean of such distances on both sides of the railway.