There is optimum way to prepare charcoal pellets that adsorb radon very efficiently. You can consult old issues of "Exploration and research for atomic minerals" published by AMD, DAE, India.
There is no "optimum" way. If there is a surge of Rn influx at the beginning of a typical measuring period you lose this information after several Rn half-lives. The optimum way is to measure by means of integrating device (not charcoal) over whole year, to avoid the variations during a year. Well, even that is not reliable because there are variations from one year to another.