I use 1/2" NPT thread stainless needle valves. Concentrate comes out of the membrane and into a tee. The concentrate goes into each needle valve - one to the flow meter and the other to the recirculation line back to the high pressure pump. Start the RO with the valves full open and close them down together to get the pressure and concentrate flow you want. I find that I have to fiddle with them initially to get close to a normal operating pressure of 200-250PSI. That is, you start closing both and balancing system pressure with the recirc valve as you close the concentrate valve until you are at the desired flow. It's just a balancing act at first because as pressure rises you will have to keep dialing back the recirc valve (open it) as you dial in the concentrate flow. At that point the concentrate needle valve will finely adjust the concentrate flow and the recirculation valve will finely tune your system pressure. Works great!
I typically concentrate at 50 gph to keep up with my evaporator feed rate and permeate runs around 0.9-1.2 gph depending on temperature and membrane condition. Using a single XLE-4040 you can slow this down enough to get 7-8 brix concentrate at 11-13 gph. The rest of the fluid is being recirculated and that's why you can do it this way in one pass. It's just much slower.
I got my valves from https://www.directmaterial.com/stain...lves-10000-psi