I use a cal-mag with feeding for coco. Used the epsom salt as a foliage spray when I initially saw the issue. I use R/O so not sure as far as amounts of cal-mag, but it's kind of hard to use too much cal-mag unless you go crazy. You can use epson in water and nutes. Same amount (1 tsp per gal). It will only help with mag not calcium. Coco seems to need a regular cal-mag regiment. Don't know the specific specs of your nutes, but most require some additional cal-mag. Not sure if your tap water mag (11) is ppm or what specific mag ppm your water is already supplying vs whats required. If your final PH is 7.40 you need to lower it, as you may be blocking out essential nutes. With coco around 5.8-6.0 is good. 7.4 may be fine for mag but may block out other nutrients in hrdro. I'm a firm believer that varying the ph somewhat is a good thing. You can use ph down or silica to lower the ph. A simple search for ph nutrient absorption charts may help.