Long story short: Just after vacuuming up a bunch of pollen, I blew an electrical switch and had no pool pump for 4 days until I could get someone to fix it. (I don't mess with electricity and water.) My pool turned green, so I brought it up to mustard algae level for 24 hours (just in case). I did this by assuming FC was 0 and adding 12.5% chlorine according to ThePoolCalculator, once in the morning, then again at night. (No, I did not test at night, but I added the same amount both times.)
This afternoon, I let the FC drop to regular shock level. I tested the water, and it was exactly at shock level (15) ... great! But what threw me is that I had just 0.2 for my combined chlorine! I expected it to be higher, so I waited an hour and tested again -- exact same results (15 FC, 0.2 CC).
Have I actually killed all the algae, even though my pool is still green (getting better slowly)? Or is my reagent bad?