check the picture below.
so, if we take half of 12½, we end up at the middle of the wall, then we take half of the width of the cabinet, in this case 1.55 as you see in the picture, and we subtract that amount from the half of 12½, that's where the left-edge of the cabinet will be at.
so, 12½ or 12.5 divided by 2 is 6.25, that's the middle of the wall, and then we move it over by 1.55, so 6.25 - 1.55, or 4.7 feet.