Don't go below an FC that is 5% of the CYA level with an SWCG. So if you are really at 60 ppm, don't go below 3 ppm. If you're at 80 ppm, don't go below 4 ppm FC. You risk getting algae if you go too low and it won't necessarily be visible but will instead look like an insatiable chlorine demand where you have to increase your SWCG on-time as it is just barely able to keep up with killing nascent not-yet-visible algae. This is where the myth of high phosphate levels "eating" chlorine or preventing SWCGs from outputting chlorine came from -- it's just nascent algae growth that is as fast as the relatively slow rate of chlorine introduction from an SWCG. Having the FC/CYA ratio be high enough to kill algae faster than it can grow is the answer; hence the 5% recommendation (for a sustained level; if you've already got nascent algae growth, then shocking would be needed to get ahead of such growth first).