Genetic, type of the substrate, water quality and feeding frequency, size and age of the plant, reletive humanity, air temperature, CO2 level and even more, all effect how much nutrient you should use.. Thats why feeding chart at the back of the bottle is just a recommendation.
You use Ca-Mg because flora trio has been formulated for low CEC substrate not coco(Ca-Mg tries to correct Ca/N ans Mg/N). generally DTW systems need lower nutrient level than recycling system. This is the main reason.
If you use coco in an open(DTW/RTW) system, follow Nebula.