my system is a 4040 membrane diy'd by myself but guess the flush valve on a pure freedom would be the same principle. my valve is a john guest type, when fully open its inline with the hose, fully closed its 90 degrees to that. when I produce pure water the valve is set at 45 degrees, so 50% waste and 50% pure.
An ro unit should never be run with the flush valve comletely closed, it must produce waste water !
Apologies if I've just stated the obvious and you were already doing this
Simon