Every time I water, I add 1 tablespoon per gallon of water and moisturize each plant accordingly... taking care not to overwater... cannabis likes moist (not wet) soil... especially when they are very young (1-4 weeks of age)...
Epsom salt in the water (all the time) along with top dressing with a cup of oyster flour or bone meal mixed with a nice compost at 35 days will eliminate the need to use CalMag and is much cheaper...