Hi Oliver,
10ppm would fine, even a little more would be ok, I wouldn't feel like i was cheating have a look at this from a U.S. site that supplies pure water systems for cleaning carpets,
Why Pure Water Rinse?
Pure water, unlike soft or hard water, has virtually no dissolved minerals and requires no surfactants or conditioners to overcome hardness or other dissolved components. In standard cleaning processes using traditional chemicals and local water, the residual minerals in the water combine with chemicals which then attach to surfaces and fibers as they dry. After drying, the residual minerals and chemicals are seen as visible contaminants or can flake off fibers and become airborne, similar to cat dander, Airborne mineral and chemical dust triggers allergic reactions, irritates skin, eyes, nose, throat and lungs.
Indoor air quality has become a significant health issue. Only a small amount of our all natural chemical (if any) is required with a Pure Rinse carpet cleaning process and following its application with a pure water rinse will dramatically reduce, if not eliminate, a major source of indoor air pollution. In addition, carpets and upholstery stay cleaner longer, feel softer, maintain appearance and have a clean, fresh aroma.
all the best
John