Everyone has a different sized thumb,
but in general folks like Medsen Fey want to be absolutely sure that nothing can ever re-ferment once stabilized. He wants to add enough to absolutely prevent any yeast, including the typical wild spoilage yeasts found in sub-tropical Florida. He thus uses 200 ppm as his "equivalent sorbic acid" addition. That becomes roughly 267 mg/liter, or approximately 1 gram per gallon, of potassium sorbate.
My thumbs must be shorter, because I'm generally happy with 100 to 125 ppm equivalent, which is more like 0.5 to 0.6 grams sorbate per gallon.