I would estimate your pH at 7.5- split the difference, lol
No matter what you'll need to change the parameters slowly, so my 2cents would say just do say a 10% PWC daily replacing with RO water. Wait an hour and test your pH and hardness, see if you can measure any change. Keep doing this till you get where you want to be. If you do it slowly and space out the RO additions, then you'll get a good idea of how much it takes to make your parameters drop, which will also give you a good idea of when to top off with tap water vs RO water in the future.
IMO this is the simple way to do it.
The more complicated way would involve doing the math and always keeping premixed RO/tap water on hand for each PWC... I'm too lazy for that
