If you test the pH of pure RO, you may find it varies significantly from your current pH. But... if it's good RO water, its buffering capacity should be near zero. The ion balance in the remaining tap water should work to keep the pH at or very near its current level. The concentration (KH) will go down, but the balance between acid/alkaline ions in the mixed tap/RO water (and thus the pH) should not significantly change.
If your pH is 8.5 with pure tap water, which I'm assuming has pretty high GH/KH values, a 50/50 mix will also have a pH very near 8.5 and GH/KH values about 1/2 of what your pure tap water has.
If you're wanting to lower your pH, it will be easier to do with a 50/50 mix. You won't need to add as much acid. If you want to do that, I'd suggest using an acidifier made for Discus tanks. They contain no phosphate. "Amazon Acidifier" or "Discus Buffer" are two examples. They use HCl (hydrochloric acid) to lower the pH and contain no phosphoric acid.
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