For sunlight loss, the chlorine that is unbound to CYA comes in two flavors, hypochlorous acid and hypochlorite ion. The hypochlorous acid has a half-life in direct noontime sunlight of 2 hours and 10 minutes while the hypochlorite ion has a half-life of 35 minutes. At a pH near 7.5 with a 50/50 mix of these two, the overall half-life is 35 minutes. With CYA in the water, most of the chlorine is bound to CYA (97-99% of it depending on typical FC and CYA level). The chlorine bound to CYA has a much longer half-life on the order of many hours and there is some shielding effect of UV absorption by CYA (or chlorine bound to CYA) that may also be occurring.
In an uncovered pool with low bather-load as is the case with most residential outdoor pools, most chlorine loss is from sunlight. If you have a mostly opaque pool cover, the chlorine loss will be lower, but will still occur since chlorine will slowly oxidize the cover. This might be as much as 0.5 ppm FC per day (depends on temperature and cover material).
There is also a slow chlorine oxidation of CYA itself, but this may account for only around 0.2 FC per day.
A higher bather-load will consume chlorine, but one person-hour in your 20,000 gallon pool would only consume 0.05 ppm FC. So you would only notice this form of chlorine loss if you were to have a pool party with a lot of people in the pool for a while or if you had urinating kids since one cup of urine is roughly equivalent to 5 person-hours in a pool.
If you get blown-in pollen, leaves, and other organic debris, then that will consume chlorine. If you let the FC/CYA ratio get too low, then algae can grow and that will consume chlorine.
All of the chlorine demand except for that lost from sunlight is temperature dependent so the rate of loss is faster at higher water temperature.
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As you can see, my CYA is low, so I can easily raise it to whatever I need. But, of course, I don't want to raise it too far since it's hard to lower. A CYA of 50 would protect the FC from so much loss due to sun, but it would also require a higher minimum FC level, which would encourage more bleach usage.
You are missing a critical point and that is the CYA (or chlorine bound to CYA) shielding effect that is non-linear. This means that you actually lose less absolute amounts of chlorine at higher CYA levels even proportionally raising the FC level to keep the FC/CYA ratio constant. In other words, you will lose less chlorine with 5 ppm FC and 50 ppm CYA than you would with 3 ppm FC and 30 ppm CYA. The only reason you wouldn't want to go higher than 50 ppm CYA unless you had to (i.e. were in a very sunny area such as the desert or deep south with intense sun) is that you'd need a lot more chlorine to do a SLAM if that were ever to be needed.