Are you using some magical bleach this evening?

Well, let's back-up and see what you have here:
- Starting at an FC of 2 and adding 3.5 gal of 10%, it should've taken you to an FC of 17 (per Poolmath Calculator) If it only went to 13, either your 10% was weak, your pool volume off, or testing was in error (10 ml sample .. use drops until pink goes to clear and multiply by 0.5 - basically but in half totally # of drops.
- 2 more quarts & 3 cups should take the FC to about 20
- No more additions but still climbing to 24 ???
So I suspect a various things may be happening ...
- Are you testing with the pump mixing the water real well? If so, are you testing from the same location each time?
- Based on the total amount of 10% bleach added, it's very possible your # (24) and mine (20) are separated because of either pool volume, testing disparities, or strength in bleach. Or combination of them all. Really hard to tell.
Oh, on the calculator, did you manually enter "10" in the field on the FC row titled "weight" for the percentage of the bleach?