As John says, with an SWG you can go lower than the minimum in the chart. Around 3 ppm FC for 60-80 ppm CYA seems to be about the low limit. 2 ppm FC or lower produced algae in several SWG pools. A very few pools even had algae at 3 ppm and needed to use the minimum FC in the chart, but they are the exception. I think that the superchlorination in the SWG cell over multiple turnovers plus the consistent dosing prevent the free-floating green algae from growing.