what are you on? dwc? i wonder since the plants are always in the nutrient solution in a dwc that it needs MUCH less nutes. I can't even get close to 4.5 EC.Think this one is too hard to answer. Depends on strain , size of plant, temperature and heaps of other variables. Start off low and pump them up slowly till you notice tips burning. Mine that are around 4 weeks flowering are at 4.5-5 EC without burning