Since you seem to be dusting because of the requirements of the fair, contact the people running the chicken show at the fair and ask for further details about that requirement. They may have something about how often, they may want it a certain time before the show. It may just be cover the butt verbiage, as long as you do something they don't really care what or when.
I never dust mine unless mites or lice are present, which means I have never dusted mine. Due to their life cycle, if you do have them it takes at least two treatments to get rid of them. First treatment gets the live ones, second treatment gets the ones that hatched after the first dusting but before they lay eggs. Depending on the type of mites you might need to treat the entire coop.