Plug all your numbers into poolmath and set your target pH. It will give you a dose based on the pH and TA. Then scroll down to the bottom to Effects of Adding Chemicals. Input the dose of acid there. The pH reduction won;t be an exact match, but close, but the TA reduction has always been right on the money in my experience. That should give you a close approximation of what the new TA will be.
Also be aware that if pH reads 8.2, there's a good chance it's actually higher than that, but you've reached the limits of the test kit.