How to properly premix and distribute makeup water for auto-WC

I’m sure its around here, but what has been found to KISS method an auto overfill style auto WC when you want to do 4 parts RO water, 1 part carbon filtered tap water as your ultimate makeup?

Quick thoughts are three separate containers, 1 dechlored Tap, 1 RO and 1 for the final mixing vessel to distribute to tanks.

Planning on supplying the juveniles with direct dechlorinated tap for efficiency/cost/hobby preference, but I need auto-changes breeder water and our tap is 8.1pH, ~6-8dKH, ~4-500ppm GH

What does valving design look like and does this ~7.4pH, ~1dKH, 70-80ppm GH sounds like correct water for breeding most domestic discus in a fish room set up?

Please feel free to dissuade me if my thoughts are way off.