Re: How Much Borax?
I calculate that it would take 60.8 ounces weight (3.8 pounds) of Soda Ash / Washing Soda / Sodium Carbonate to go from a pH of 7.0 to 7.4 and that this would raise the TA from 100 to 128.2. It would take 120.4 ounces weight (7.5 pounds) of 20 Mule Team Borax to make the same change in pH, but the TA would only go from 100 to 115.5
Just multiply whatever weight of Soda Ash is required by 2 to get the amount of 20 Mule Team Borax that is required.
I calculated that using 4.7 pounds of Soda Ash would raise the pH from 7.0 to 7.53 so that really isn't that far off.
Your CYA level is very high even for an SWG system (they usually recommend 70-80). You are right to keep your FC at 5 ppm with that high a CYA level. I am surprised, however, at your low pH since an SWG system normally shows a rising pH. You also show having an Ozone system -- why is that? With an SWG there is very little need for an ozone system since it will just end up breaking down chlorine faster requiring you to run your SWG longer. The SWG should be more than sufficient at eliminating any combined chlorine. Are you adding non-chlorine shock (potassium monopersulfate) regularly or do you also have a floating feeder or other source of Trichlor tablets/pucks? If so, then that might explain the low pH and the high CYA (and both of these items, non-chlorine shock and Trichlor, are unnecessary with an SWG).
Finally, you indicate that the chlorine level is at 5, but what sort of test kit are you using? If it's a standard OTO or DPD color comparator kit, these usually only show a maximum of 5 ppm FC so your actual FC might be far higher. If you don't have one already, I suggest getting a Taylor K-2006 drop-based FAS-DPD chlorine test kit that will also test pH, TA, CH and CYA. I wouldn't necessarily trust your pool store's measurements if that is where they came from.