When and How to Enter Weight

In order to make the best use of the recommended burn meter for kids (http://www.bcm.edu/cnrc/bodycomp/bmiz2.html), what time of day's weight should you enter?  Does it assume you're wearing clothes for its estimate?  I've heard that the best time is naked in the morning, but the only scale available to me is in my parents' bathroom, so... is doing it clothed in the afternoon still going to give me a good burn estimate?