I was curious to whether or not I could flush with shower water and then on the last round add RO water to get rid of any impurities coming from my shower water... I ask this because I have a bit of pots to flush and using all RO water would be way to much of a hassle since I only have two, five gallon jugs that I have to take to the store to fill.
Yes .. that's a workable plan .. after a couple of flushes the soil will be at about the PPM of your tap water - and that may be good enough if you have reasonable soft water .. but a final flush with RO would be a great finish. Remember - the soil itself may be adding a PPM load.
Tell the truth - when I'm flushing pots I monitor the PPM of the runoff and am not fixed on the idea of a 3X flsuh - when it gets close to my inflow I call it good...