Neil what system do you use ?
Ro Man 300 gpd system which on it's own wouldn't produce the amounts you need.
You could plumb several of these in to produce enough if need be. But I think you'll need a dedicated system for your amounts, although I think Trevor Knight runs a few Ro Man systems.
Currently tap water is 270 ppm in summer goes to 350
Product water 5ppm which we then DI into the tank.
Product to waste is 1:1
In the winter the waste goes down the drain, in the summer it waters the garden.