[Delphinus]

Looks like you have your answer already but since this is a topic I feel I have a tiny bit of knowledge on, I'm compelled to post anyhow.

Yes 0 TDS is possible in Calgary water with RO alone, but typically it's better still to use the DI because there tends to be TDS surges and the DI will catch those. It depends on the membrane, some brands have better rejection rates than others. With the membrane I have right now, the best I can get after a 20 minute flush is about 3-4 TDS (it will go down to zero after about an hour or so). The membrane I had before had such a great rejection rate that it would be zero TDS within a 2 minute flush. I've kept my old one so I can remember what kind to order when I've had enough of this one.

Keep an eye on your RO output TDS, when it stops going below 10 on a 5 minute flush it's pretty much time to let it go. Expect to replace them around 12 to 18 months around here.

If you don't have a flush kit, get one.

And and whatever you do, even if you have a auto-shutoff on your RO, don't hook it up to a float valve or float switch in your tank, because constant on/off's are the fastest way to kill a membrane. They are designed for long runs, so turn it on, flush it for 2-5 minutes or whatever, then let it run for say 12 hours. Shut it off and leave it off until you need more again.

The ONLY thing those shutoff kits should be used for is preventing overfills. As soon as whatever it is you're filling is full, turn off the source water.