From what I can see is you have them in 3 gallon pots, Watering half a gallon twice a day. Half that can be water suspended in the pot so 1.5 gallons of water/nutrients are in the pot. That 1.5 gallons is EC of 590. When you add 0.5 gallons of 1200EC water to the pot it mixes with the 1.5 gallons of water already in the pot and would only increase it to 600 or so. I agree with
@Hashy you don't want to go any higher then where you currently are. Less is always more and if they look happy.