Your height and weight are above the world average - certainly six a day would make sense. And, figure that when they give the maximum dose they are taking into consideration that there are plenty of people, particularly women, who are quite petite.
It is magnesium in the cleanse, not manganese. Magnesium deficiency is extremely widespread due to many things - fluoridation of water, damage to soils by modern farm techniques, and our modern diets.
Magnesium oxide based cleansing products require an acidic environment in order to be activated. HCL is hydrochloric acid - stomach acid. An empty stomach in a person who produces enough HCL is quite acidic, but some people do not produce as much HCL as would be beneficial. Taking your oxycleanse on an empty stomach with a couple tablespoons of apple cider vinegar, lemon juice, or HCL tablets would ensure that enough acid is present to activate the products cleansing capabilities.