2. For the purposes of paragraph 1 above the estimated life shall be calculated by dividing–
(a)the quantity of the relevant mineral remaining unworked in the site at that date, being the amount which, if the site were sold in the open market, a willing vendor and a willing purchaser might be expected to agree as being economically workable and saleable mineral reserves; by
(b)the anticipated average annual sales at that date of that unworked mineral, being the identical figure arrived at as under paragraph (b) of Schedule 1.