And keep in mind that there are just as many theories of where to hook it up as there are meters.

In my RO/DI unit I chose to test the output of my RO going into my DI and coming out of my DI. That way I know and recorded my initial RO output. I can track it and if it begins to creep up I know to change the sediment filters. If it rises quickly the membrane is probably shot. Then I test the DI output. If it ever creeps up from 0TDS then the resin needs to be changed.

In an RO only set up people tend to measure going into the RO membrane and coming out of it. That way you know for sure if your pre-filters are going or if your membrane is bad.