If you're under a court order to account for it, which can and does happen when a court has questions about how the money is being spent, don't tell anyone to "pound sand." If you're not, it's not unreasonable for someone to want to know where their money is going. Becoming combative just because the system enables you to do so is going to breed nothing but contempt down the line. In short, if you tell someone to go pound sand on a reasonable requestion don't be surprised down the road when you need some extra help to spend the little one to basketball camp, buy a new computer, or what have you and your ex tells you to go pound sand because he can.
The most accurate way is just to factor the value of the child's room. As an adult you would still need all the other space in the home, so the only actual expense of the child are his/her quaters. A reasonable share of utilites depends on the age of the child, probably about a 25% share and the same (or less for very young children) for food.