An open office floor goes about three times as fast as a washroom of the same size. That is why you set a rate of work per room and not one feeling for the whole building. Square metres divided by square metres per hour gives the hours; nothing more clever than that happens here.
A month is not four weeks but 4.333, because fifty two weeks divided by twelve months is 4.333. Over a year that difference is one visit in twelve, and on a monthly contract that is real money.
