Some like the meters, others find them a bit questionable due to calibration issues. To each their own. In your case, we can safely assume the pH is at least 8.2. Using the
PoolMath tool, with an assumed water temp of about 75 (wasn't sure), I would add 1.5 cups of muriatic acid to your 21.6K pool. That would ideally lower it to about 7.8, but you aren't completely sure how high the pH actually is. So give it about 30-60 minutes then test again and continue that process in stages until you feel comfortable you've reached a good pH goal. A pH of 7.4-7.6 gets you to a slightly negative CSI.
Keep an eye on the TA. The acid additions might pull the TA down as well. Don't go any lower than 50.