I don't think you'll have good luck with just dumping potassium sorbate in an active fermentation and expect it to stop fermenting. It doesn't work that way.
Sorbate doesn't kill yeast, but inhibits yeast reproduction. The yeast won't need to reproduce at that point, as there will be a hundred billion of them so sorbate won't do a thing.
You could try crash cooling to halt fermentation, then racking off of the sediment once it falls out and doing it again and THEN adding the sorbate.
Or you can let it ferment out, rack off of the yeast once it's completely clear, and then add the sorbate and a couple of days later you can sweeten to taste.
I'm going to start my lemonade at 1.070 or less, let it ferment out, then stablize and sweeten.
Or, instead of adding any sorbate at all, chill it when it's where you want (I like it at 1.010-1.020), keg it when clear, and don't allow it to ever warm up.
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