Watermom, as another newbie I could use an explanation or two of the Best Guess chart.

first, what are the two columns labelled "shock"? (meaning, what is the difference between the two)?

second, could you explain why the very high FC levels would call for shocking with more chlorine? This doesn't make sense to me, but then again I'm new, so very little pool stuff makes sense to me just yet.

Thanks in advance for any insight you can provide.