That was the quick answer, of course you need to make sure the water temp is at 65'f and it could vary slightly because of your water makeup. But pure bottled water at 65ish is pretty much spot on 1.000 on the scale. However I think they calibrate some to 60'F and some to 70'F, should say somewhere on the scale itself, or its paper insert that came with it.
Even the hardest drinking water will be less than one point heavier than pure water. By way of contrast, sea water would give you a reading of 1.025 and that has 35,000 ppm of minerals.