would like to add one more question to Neils excellent post....what is meant by "topping off" ? Do you add x amount of water, water + PO? Sorry, this is probably clear to everyone but me...I understand if you made up 1 liter of PO, after a period of use, it could be reduced down to say 950 ml, would you then add water back to equal 1 liter, or would you add 50 ml of water + x grams of PO?