Sounds like its doing its job. You will want to test the TDS in three different spots to really know how well it is working though. Since the RO membrane does the bulk of the work you need to know what the TDS after RO only is. If it is not performing up to snuff then DI life will be short, especially with a tap water TDS over 400. If you bought the Premium system it will have an RO bypass valve installed so grab a sample from it for that number.
Remember to always rinse the meter and whatever you are testing in, be it the cap or I prefer a clear glass drinking water glass myself with no soap residue or water spots to interfere with TDS readings. Take the tap water sample first, let it stabilize and record the reading. Triple rinse the meter and glass in DI water the take the RO only sample, again let it stabilize and triple rinse. Finally test the RO/DI effluent and do a final rinse so you put the meter and glass away clean for next time. Always turn the glass upside down to keep nasties out.
You should be seeing a 96-98% reduction in TDS with RO only and 0-1 with the RO/DI. When you first start to see anything other than 0-1 consistently on the DI its time for a resin change. DI releases weakly ionized substances back into the treated water so don't let it go too long.
