Once you add the drops and mix it well, go ahead and read your result. That is going to be your true number. Don't worry about what it looks like after being allowed to sit. Regarding the residue, I'm not sure what that could be except maybe some of the iron or other "stuff" in your water collecting there. You mentioned that your DH had been doing some landscaping, it's possible that some of that debris has blown into the pool and collected in the skimmer. I occasionally get a dark-grey to blackish line inside mine, but that's from dirt, suntan lotion, sunscreen, sweat, and all the other gunk that gets introduced from the kids. Usually a baking soda/water paste and a sponge will get it off, though.

How did the filter look this morning?

Janet