Have to admit I never flush unless I remember to do it manually despite having an auto RODI ATO top off system running every 10 gallons or so. My input TDS is only around 70 though and the DI resin lasts over 6 months.Try the BRS water saver upgrade if possible for your rodi unit. You basically add a second membrane and it cuts your waste pretty much in half. Also I run an auto flush system and was shocked to see how much that wastes. I still run it as my water is automated but if I manually made rodi water I would ditch it. The ratio of gallons made to gallons wasted published by most units does not account for flushing