As Gail said... just a pinch! Just make sure the feeders are "lightly" dusted.
In case you don't know how often to give supplements...
Young beardies... calcium 5 x a week(only dusting ONE feeding per day).. multivitamin 2 x a week(only dusting ONE feeding per day)
Adult beardies... calcium 3 x a week and a multivitamin 1 x a week