[Amazin]

I'm wondering why, why not  e.g. 70/30pure to waste ratio so we get more pure out of it. I'm guessing that's bad for membrane?

[Simon Trapani]

Gotta find your sweet spot. Ideally via a gate valve for your lowest tds out, whilst still having enough waste flowing over the membrane so as not to clog it. Also negates the need to flush.

[Spruce]

You find where your own sweet spot is by slowly adjusting the gate valve further open from 60/40 or fractionally closed from 60/40.
The sweet spot is when your pure leaving your r/o membrane is the lowest.
You make a slight adjustment and then wait for the tds to settle before taking a reading.