A pH of 7.9 is withing normal range, I think. It is only slightly basic/alkaline. You don't want water that is acidic, as it will rapidly eat plumbing and water heaters. So 7.9 better than, say, 6.1 (7.0 is neutral).
In any case, STPP would tend to raise the pH above 7.9 anyway. Most detergents need some alkalinity to function well. Alkalinity helps to clean greasy soils - it literally turns the grease into a form of soap.
A soda taste to the water isn't so much the alkalinity - that would taste bitter - but rather high sodium content.
I'll let someone else figure the ppm to grains. It's been a while since i looked it up.