You could, but you see when you add salt mixes, trace elements, and well the term TDS, Total Dissolved Solids, describes it. So it wouldnt do you much good, because your salt mix is a solid dissolved in the water. Literally any solids that are dissolved in that water count. You would get a number but no way to discern what solid is what part of that number. Salt, Calcium, alkalinity, etc.
You want those, you dont want the ones in tap water. Like phosphates, metals, maybe eve soils I dont know. Not good, I know that. Silicates. Frankly, not even for drinking in my opinion.
In fact, just to tow a tiny true story into this post real quick, the local store based RO machines run about 11. Whereas mine runs 0, even just starting it. Though it's been my understanding that a true 0 may be hard to actually come by. Though my incoming is at best 70 so...
