There have been numerous "discussions" here on the virtues of TSP vs. STPP. Just to say, TSP (trisodium phosphate) generates a precipitate when binding with minerals in the water whereas STPP (sodium tripolyphosphate) does not. Washer manufacturers back in the day advised to add a non-precipitating water softener with soap to combat the effects of hard water and avoid scum. Washday miracle-detergent products included it, until phosphates were eliminated.
The dosage is whatever amount "breaks" the water (makes it feel slippery) before the detergent is added.