Consider the amount of fuel on board at departure.The Amount of fuel it would take to reach the last know position.Subtract the large amount of fuel it would take to climb to and maintain 40,000 ft.At the last know position calculate the distance it could fly with remaining fuel and then from that last know position draw a circle with a radios of the distance the plane could fly on the remaining fuel left after all the calculations.The plane will be somewhere within that circle or very damn close.