When filling the chlorine only tube, the water should line-up with the 10 ml marking. Unlike the OTO block viewer, there shouldn't be an obvious curve at the waterline sample, but if there is, just go with the bottom point of the curve. But in the grand scheme of things, it shouldn't matter that much. Same with the power - one generous scoop of powder for a 10 ml water sample.
The fact that you noticed a 1 ppm difference in test separate tests performed at about the same time, says something is inconsistent with the testing. I'm assuming the water sample was from the same batch, so perhaps there was a counting error? It gets monotonous counting so many drops of about 40 or so.
But let's give you the benefit of the doubt and say your morning FC was 19.5. You started at 21.5, so that's still a 2 ppm drop in darkness. At this stage it really doesn't make sense. I know you're exhausted from the
SLAM Process, and you close Monday. So here's what I would suggest:
With pool completely void of any toys or anything that could visibly hold algae somewhere (no niche, drain cover, etc), do a final FC increase to Mustard Algae level of 29 (based on a current CYA as the guide). Just add enough chlorine to get to 29 and call it done - no swimming while that high. It will take a couple days for that FC to fall. By Monday the FC may still be slightly elevated, but should be below SLAM level and ready for closing. Take a well deserved break and relax until next year.
If out of curiosity you want to do another
Overnight Chlorine Loss Test, wait until Sunday evening/Monday morning to try it. I'd be curious to see what you get.