Matt - you're making it way to complicated. To get the pH just right, you'd have to know the alkalinity also (buffering capacity). I have a pool also (~18,000 gal - 68,130 L) and when my pH gets to 8.2, I only have to add about 1/3 gal (~1.26 L) of muriatic acid (my HCL is about 10.2 M) to get it back down to about 7.5. I would start by adding about 1.5 L of acid, let pump run at least 24 hr, then check pH. Then add more HCl accordingly to sneak up (or down) on it. Key is to mix (pump) for at least 12-24 hrs before adding more HCl. If you don't mix well enough in between additions of HCl, you could get too acidic and cause other problems. Be patient.