looks like my last round when i got ph out of balance in soil. EC 700/ppm 350 will do at this stage.
If you don't have the values of Ca,Mg and N in your water, its most likely that if you live in a big city, you'll have hard water and if you live in the mountains it may be very soft. Soft water you can start with 1:1 mit with ro/destilled with hard water I'd start at 3 (dest):1(tap) and work up.
I always add some epsom salt too, to get the balance of Ca/mg.
Ever since I do that, i never had a cal/mag problem and no waste of money. But I have to mention that there is N in the water of course, but not too much. I live where tap quality water is regulated and has controlled quality. There are only some countries where I would advise to drink water directly from the wall. And theres no chlorine in the water too - so many problems I don't have.