A booster pump... If your producing the water at home, dont forget when you've got things like washing machines going, showers, etc this will lower your pressure even more while there on. Ideally you want the pressure around 80psi to have your membrane working at its best a booster pump will do this.
Also what you have your waste to pure ratio set will affect your tds readings. If your mains input is around 260ppm i would personally set it at around 60 -40 pure to waste as a starting point and go from there.