You may want to look at replacing your membranes with a higher rejection rate membrane, you are just over 96% rejection. 400-15/400x100=96.25%. A 2% increase in membrane efficiency doubles the life of your DI resin without any other changes. My membrane is averaging 99.35% rejection still after two years of heavy use in Phoenix with a TDS between 500 and 800. Replacing the membrane may be cheaper than frequent DI replacements, I found this to be true myself. My DI life went from 150 gallons per 20 oz cartridge to over 830 gallons on the very first cartridge and its been over 1000 gallons per cartridge ever since. I used to have a off the shelf 75 GPD Dow Filmtec membrane which was giving me 13-15 TDS and now the MaxCap membarne does 3-4 TDS with the same source water so DI lasts much much longer. There really is a difference.