One thing to remember is that, just because the FC dips a little bit, doesn't necessarily mean that you pool will turn into a green swamp. It can takes days of clouded water before anything green ever shows up.
Here's a little personal experience - all last year (and planned for this year too), I ran my pool at 90ppm CYA and about 3ppm FC. My FC/CYA ratio was much less than the recommended levels. Guess what? Not a single day of clouded water or algae,
ever.
Now, what I did not tell you is that I have borates in my pool water which acts as a mild algaestat (it inhibits algae growth) AND I treat for phosphates every season (my current PO4 levels are 0ppb). Both of those conditions make my water much less "reactive" to algae, that is to say, even if algae found it's way into my water the low nutrient conditions (low PO4) and presence of the algaestat (boron levels ~ 50ppm) mean that the algae can not efficiently grow. In real terms that means that algae replication rates are slower giving chlorine much more margin to act as a sanitizer.
Algae and pathogens (bacteria and viri) are not destroys in a binary sense - if you have X amount of FC, everything is dead, otherwise it lives. Sanitizers are defined by their CT Kill times (concentration multiplied by application time) - if you have very high sanitizer levels, then the application time can be short to achieve a 4-Log reduction (99.99% reduction). If the sanitizer concentration is low, then it take longer times to achieve the same 4-Log reduction. This is because you have two rate processes going on - you have the kill rate of the sanitizer versus the replication (reproduction) rate of the pathogen. So, there are two levers to play with - either ramp up the kill rate so you overwhelm the pathogen and destroy it OR reduce the reproduction rate of the pathogen so that a slower kill rate of the sanitizer is still effective.
TFP Recommended levels ignore the second part of that balance and just assume the reproduction rates are what they are. So the Recommended Levels are based on achieving the best possible CT kill times without compromising bather comfort. My pool water looks to optimize BOTH sides of the equation and thus allows me to operate at a much lower chlorine level, which I find more comfortable and "easier on the wallet"....
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Looking at the first logarithmic graph from the sticky above on Pool Chemistry one sees that the level required for a kill rate above the propagation rate of bacteria is a FC level of approximately 9ppm with CyA of 70ppm. this is greater than 10%.
The actual CDC minimum disinfection level for water, as measured by ORP, corresponds to a hypochlorous acid ([HOCl]) concentration of 11ppb (0.011ppm) of HOCl. That corresponds to a very low FC/CYA ratio. The levels you see in the chart are based on anecdotal observations of what "seems to work" in most pools. Think of it like a statistical quantity and that's your mean value. There's lots of pools on either side of the mean.