Your doing exactly what I did in the beginning. You can't use 12.6 ounces in your calculations, you have to use 9 ounces. 12.6 is the finished products weight.
Their will however be a little extra Miracle-Mineral-Supplement left over using 4 ounces of water in the formula. You could use something like 3.5 ounces of water in the formula instead of 4 ounces depending on how much the volume will increase after adding the sodium chlorite.