120(12)/.1*.1(X)=120~0.1/120 figure out what X is will determine how many WC is needed. :lol::kidding:
I aint no mathamethical genius but i dont think u will competely replace all 120g with ro/di water since each wc will only dilute 10% less tank water with ro/di water. as long as u keep improving the water condition of your tank with each water change, your tank should continue to do better.
Exactly what reefman said (not necessarily the math part ). Even if you change 100,000 gallons of water you'll never be at 100% RO/DI. You will continuously dilute it and it will keep getting better for your system though.