In your example, it would be safe to swim up to FC=12. However, as was said, you nbetter trust your CYA test, and not rely on a pool store CYA test to give you this number.
Personally here is my cheat factor. Because I hate the CYA test, and can never decide when the dot is gone, I error on the side of caution. So I drop one level based upon what the test told me. But not for day to day maintance. I will give an exact example, because as I type that, I don't think it makes tons of sense. and woudl hate to confuse someone reading this.
I do the CYA test, I get CYA=30. So I keep the FC between 4-6 on a day to day basis. If I were to need to take the pool up to high levels of chlorine, the chart indicates that it would be safe to swim up to 12. However, I read the line below where CYA would be 20. For CYA=20, the SLAM level is 10, which would be my safe swim ceiling. Again, I only do this to error on the side of safety, as I hate that test. And I have even bought the 50ppm standard, and I can never decide if it if 50, 40 or 60 on the test. I think my eyes are broken. Any sane person would probably stick with the line at 12, and just leave it at that.
