I remember there was at least one SWCG pool owner who tried to use 3 ppm FC with 80 ppm CYA, but that wasn't enough to prevent algae growth. So the minimum recommendation is closer to an FC that is 5% of the CYA level (rounding up from my original 4.5%) so 4 ppm FC with 80 ppm CYA or 5 ppm FC with 100 ppm CYA. This is already lower than Ben's Min. FC for manually dosed pools, but going any lower than this may risk nascent algae growth.