Re: How do I use Chlorine in my Spa (or pool)?
In 350 gallons, a 1" 1/2-ounce Trichlor tablet will increase FC by 9.8 ppm, increase CYA by 5.9 ppm, and lower the TA by 6.9 ppm. As for pH, that depends on your TA level and aeration, but worst-case at a TA of 50 ppm with no carbon dioxide outgassing it would lower the pH from 7.5 to 6.9 (after chlorine was used/consumed). So it's not a disaster if you use only a single tablet.
As was mentioned, Dichlor is faster and easier to use and adds more CYA per FC. A 1/2 ounce of Dichlor will raise the FC by 5.9 ppm, the CYA by 5.4 ppm, lower the TA by 2.1 ppm, and the pH in my example would drop to only 7.2 ppm (after chlorine was used/consumed).