Choose the corner coordinates with the lowest x, y, z values, then offset dx, dy, dz from that. If you hit F3, the little orientation thingy in the middle of the screen can help you, the three 'legs' show in the positive direction for each, x, y, z, so you find the block furthest away from each positive direction within your test area.
Confirmed for 1.7.9 and 14w11b.
Try placing the coal in the bottom slot...