I run a probe incoming, and a probe after 3rd membrane. You only need one booster pump. Trick is restricting waste water out of last membrane to tune product.
I just got home and was able to test my supply water, and it is120ppm, which I assume is easily good enough for an extra membrane, but with a feed pressure of 100psi would it be enough in your opinion for a third membrane?
Also do you have an adjustable restrictor, if not how do you fine tune the restriction.