It depends on how hard your tap water, and what water parameter do you want in your aquarium.
My case:
Tap water: pH 6.8 ; kH 13 ; gH 9 ; TDS 280 (not consistent, depend on weather)
RO water: pH 6 (initial) ; kH 0 ; gH 0-1 ; TDS 8
I mix 17 tap water : 100 RO water, and the result is: pH 7 ; kH...