You could start with 75/25 and work to 50/50. I used calibrated electronic pH pen. Most of the time it showed pH of 7.0 because RO has equal amounts of nothing in most cases. This really was a false reading and as you start mixing you'll see different readings, but your hardness will be much less. In my experience i have not killed any Discus with that kind of drop. Go slow if your worried its not a race.