(a) If on such given day there shall have been in such market both a sale of any higher grade and a sale of any lower grade, the average of the declines, or advances, or decline and advance, as the case may be, of the next higher grade and the next lower grade so sold shall be deducted from, or added to, as the case may be, the value, on the last preceding business day, of the grade the value of which on such given day is sought to be ascertained.

(b) If on such given day there shall have been in such market a sale of either a higher or a lower grade, but not sales of both, the decline or advance of the next higher or the next lower grade so sold shall be deducted from, or added to, as the case may be, the value on the last preceding business day of the grade the value of which on such given day is sought to be ascertained.

(c) If on such given day there shall have been in such market no sale of spot cotton of any grade, the value of each grade shall be deemed to be the same as its value therein on the last preceding business day, unless in the meantime there shall have been bona fide bids and offers, or sales of hedged cotton, or other sales of cotton, or changes in prices of futures contracts made subject to the act, which in the usual course of business would clearly establish a rise or fall in the value of spot cotton in such market, in which case such rise or fall may be calculated and added to or deducted from the value on the preceding business day of cotton of all grades affected thereby.