When you say 'millelitres per litre' what do you mean
Sodium Percarbonate is normally a solid oxidising agent that breaks down into Hydrogen Peroxide when added to water that then produces your so called oxygen boost
On certain stains I would use ca 25grams per litre. As for using it for general coverage in a pre spray - I wouldn't bother. You can get presprays that already do that for you