If your RO membrane is fuctioning properly the RO only TDS should be 96-99% lower than the tap TDS. So, with your 585 going in you should see something like 5 to 23 at the highest froom the RO. Using my system as an example my softened tap TDS is 560-600 and the RO only is between 2 and 3 using a Spectrapure 99% guaranteed rejection rate Select series RO membrane. When I was using an off the shelf 75 GPD Dow Filmtec membrane the TDS was between 13 and 15. The biggest advantage with the better membrane is now my DI lasts 5-6 times or more longer so the membrane paid for itself in the first year in DI replacement savings alone. If you know your tap water pH and alkalinity you can calculate the CO2 content of your water which may be contributing to your short DI life but check your RO TDS first.