I've done two very full loads so far with the Megaperls Color, and both times used only about 1/8 cup. Both loads came out sparkling clean with food stains from cooking completely removed. I have yet to try the regular Persil, but figure I'll start with a scant 1/4 cup and see how that works.
Figuring 30ml = 1 ounce, the dosages I'm using are less than the boxes recommend for light soil in European soft water.
Ritchie, have you ever phoned our local water bureau to ask what our hardness is? I keep forgetting to do so.