Nice that you know whats in the water, Calcium, MG and sulfur in the water is all good stuff for the plant...(in moderation)
Watering to runoff to reduce salt buildup.... Hmm, I could go on and on with this as I'm still experimenting. I water to runoff now, but am considering doing my next grow without. Watering heavily can cause problems of its own, I'm still tinkering with this a bit. (yes, I use a perlite and light soil too)
Your probably right about tap water, mine is coming in around 200 ppms with a PH of around 7.5. This forces me to use PH down which I really try to avoid using. (it takes quite a bit of PH down to take tap water down to around 6.5 PH) All and all though it works just fine.
RO water comes in around 10 PPM's according to my pen. I add just a bit of Cal/Mag and a little Silicon to the RO water bringing it to around 50 PPM's... Adding nutrients during feed time drops the PH to a perfect level without PH down. Plain waterings with RO water requires just a drop or two of PH down to get the PH right. My tap water requires a lot more PH down.
