The Compendium of Bheḷa 8.7.23

8.7.23

kiṃcinnyūnātiriktaṃ vā jñātvā vyādhibalābalam | kuryāt , viṃśativarṣāṇāṃ tatra prasthapramāṇataḥ ||

or slightly less or more, having discerned the strength or weakness of the disease. / One should administer it: for those of twenty years, according to the measure of a prastha;