FC is gone after two days

Just in case it wasn’t understood that chlorine needs to be added pretty much daily. Going back to your original question: depending on how large your pool is, adding 21oz of chlorine may or may not be enough for one days use. But chlorine is consumed every day so it needs to be maintained above the FC/CYA level every day.
 
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Loss of 2ppm from OCLT confirms the need to SLAM. Whether you start before or after people are over is up to you since pool is clear so not a safety concern. You could bring it up to SLAM level before or wait til after.

If you wait until after people leave I would use the liquid chlorine portion of the FC/CYA Levels and make sure you are at or above target ranges for cya of 90

You will also have to decide whether to complete the SLAM at 90 or drain some water
 
And as Bperry mentioned you will want to reevaluate swg settings to make sure you are generating enough chlorine daily or are supplementing with enough liquid chlorine daily to stay at target levels, but you need to finish the slam first.
 
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