When adjusting a SWG, you want to measure the FC level in the early evening after a day of full sunlight and average bather load and adjust the percentage based on that. If you are expecting higher bather load you will need to bump up the percentage a day or two in advance of that.
The key is to not let the FC dip below the suggested FC level for your CYA level at any time. High temperatures and high bather loads can cause dips in the FC level. If you don't anticipate those dips in advance and prevent them, algae can get started and require shocking to cure. Stay ahead of the dips in FC by adjusting the percentage up in anticipation of extra pool usage and you will be fine. Or just target a higher FC level to begin with so you have more margin before problems set in.
The key is to not let the FC dip below the suggested FC level for your CYA level at any time. High temperatures and high bather loads can cause dips in the FC level. If you don't anticipate those dips in advance and prevent them, algae can get started and require shocking to cure. Stay ahead of the dips in FC by adjusting the percentage up in anticipation of extra pool usage and you will be fine. Or just target a higher FC level to begin with so you have more margin before problems set in.