Hi loublou, I too struggled with my spa for a couple of years until I got the hang of it. I also found this site and it helped me out. I will tell you how I do it.
1. Have to have a test kit for this to work. I use the K-2006, which is for chlorine, even though I use bromine because I also have a pool that uses chlorine and you can use it for both as you will see
2. When I add water from the tap to fill up the spa the first thing I do is add about two cap fulls of sodium bromine. This is to create what is called the bromine bank. Without getting too complicated, this bank is what will be used to make the sanitizer, similar to chlorine in your case.
3. After the sodium bromine has disolved in the water I shock the spa with some Chlorox bleach. What this does is it converts that sodium bromine into a sanitizer.
4. Ok, now I have new water and sanitizer. Now it is time to see where the chemicals are at.
5. Test using the K-2006 for TA, FC and CH. I use a meter to test my pH just because I don't like the color comparison thing. Also the number I get for the FC test I multiply in my head by 2.25 which is the bromine level. Since the K-2006 is a chlorine test kit you can get your bromine level by this calculation. In your case since you are using chlorine as your sanitizer you don't have to do this.
6. Now I just adjust the chemicals as needed. I keep my TA down around 50ppm because if not the pH will go through the roof. At around 50ppm the pH just hangs out around 7.5 (perfect). The CH I keep around 250 and you don't have to check that again.
7. I have a floater tied to a string that holds about 5 bromine tablets. This floater keeps the Bromine level stable when not in use.
8. After a bunch of kids get out of the spa I shock it using Chlorox because the bromine level will quickly drop due to the bather load.
9. I refill about every three months.
Hope this helps

9. Every day or two I check the pH and FC level (times 2.25 for bromine) to make sure it is where I want it to be and adjust as necessary. I check the TA about once a week although I will know if it is getting low because the pH will begin to drop when I check for it.