On my RO/DI 4 stage unit I run a 5 micron prefilter and a AC whole house filter then the 4 stage, with a second DI HS, I just got a TDS meter in and out probes. I have found when you first start making water the readings on the out let were running 27. 387 in, after 30 minutes the out let was down to 2, a RO membrane takes some time/ water flow to be totally efficient, that is why allot of people with auto top offs have problems, it is only adding a gallon or 2 and then shuts off, but the RO is not working to full capacity yet as far as removing TDS.
Bob