If your CYA is 80, then an FC level of 8.5 is just not an issue. It is always better to be on the high side than let the FC get too low.
Keep in mind your "target" is 6, but any level up to 31 is OK to swim in.. In theory, you want your FC to be 6 or above all the times.. If you quit producing chlorine at midnight and start again at noon, you want your FC to be 6 when you start producing chlorine again. Just pretend that the pool will explode if your FC ever drops below the minimum level of 4 ppm.. It won't of course, but the idea is you want to be at your target and not get anywhere close to the minimum.