The EC range they give has nothing to do with them assuming you will use additives to get it in that range, the ml per l, is their recommended dilution rate, however as said before you need to take into account solution temperature to get a more accurate ec, I am not sure what temp they recommend the solution to be. But you'll want to add nutrients till you hit that range, not add additives. I would slowly adjust upwards and not to 1.5 immediately.