Email Daqua.co.uk and ask Doug what restrictor he recommends.
Depending on the hardness of you water and whether you are on a meter or not, I would replace the restrictor with a tap so I could manually control the pure to waste ratio.
In other words, if your water hardness doesn't justify a 3 to 1 ratio, then using a restrictor at this ratio is a waste of water. If you are on a meter then it becomes a balancing act between water costs and membrane costs.