assuming 0.7EC? that'd be about 350ppm. that is hard but should not be a problem. I have realtively the same hardness. not a problem in 4 or 5 years, whatever it has been. i started using soft water last winter with no issues (logistics forced the decision, but no ill effects, think it may hav even helped with antifungal properties of plants, but not certain)
with a soilless substrate you can easily start at 1.3-1.5 EC. heck even a seedling won't burn at that concentration, but is better to start at a reduced strength the first week in my experience, and even when ramping up there's a bit of lag between what you feed and what the substrate is. Be religious with that 10% runoff waste water and substrate will not be the cause of any "buildup" unless you upped the concentration yourself ... can still happen in the plant of course. they cannot excrete waste. so any mineral you feed at a faster rate over time than it can use will cause a toxicity in plant. all about finding that right rate feeding... will vary by local environmental factors.
growdiaries.com/static/img/04a614e499ea563bf72eb4fffeda9585.jpg
i see now... it's just shifting decimal 3 spots, so easy translation. definitely stick to EC... as you can see these pens calculate ppm vastly differently due to manufacturer and nothing to do with reality. all are guesstimates on ppm - the cheap ec/ppm meters.