I'd venture a guess
that we all get a feeling for how much is the right amount over time.
In Munich, I used a zeolith based water softener and Persil Mega-Perls.
Worked great.
Here, I use TSP or STTP and one of a variety of enzyme based name brand no-scent detergents (can't stand strong scents) and double the washing time on everything. Wash at 145F, of course as the bullshit about cold water washing is total nonsense.
Everything comes out just as clean as it did in Germany and, in both cases, I use the normal amount of detergent for the quantity of clothing being washed.
My mom, who was an early enzyme adopter, lets her FL fill with warm water (120F, not 75F), tumble for five minutes and then resets the start to delay by one hour. Everything comes out clean as a whistle and she uses the recommended amount of whichever Persil is on sale.