I have been W F P since November and just getting into my 300 gal R o and the workings of that. My R O has a booster pump but will produce ooo tds after my D I quite happily at 40 psi, It’s a bit slow. So it fitted a tap between the Di inlet and the R o out let, just to monitor my results. With my tap water pressure at 40 psi the tds is 110 from a 330 tds from the tap. When is switch on the booster pump which works a 65 psi it comes down to 7 tds.I can turn my pump up to 80 psi my R o delivers a tds of 3. This proves that a membrane works so well at 80 psi and even if they say switch off your pump to stop problems of over heating your transformer, rather buy a larger transformer. The one I use starts at 9 volt through to 15 volt which the pump is made to cope, at 12 volt the psi is 65 at 15 volt the psi is 80 psi.
The fact remains one needs to get your Psi up for your membrane to operate at its optimum ability . Which in turn will save money on your D I resin costs.