[EDIT] I posted right after Watermom, but we basically are saying the same thing. [END-EDIT]

For an outdoor pool, it doesn't work the way you described because the CYA protection of chlorine from breakdown from sunlight is non-linear which means that even doubling the FC level when you go from 40 ppm CYA to 80 ppm CYA doesn't mean that you lose more chlorine in absolute amount. You actually lose less. The downside is that a higher CYA level makes it harder to solve any problems via shocking since the FC level has to get so much higher. So one usually trades off this risk by using a CYA level that is high enough for reasonable chlorine loss from sunlight. For many residential pools, that's around 50 ppm CYA or so. However, in very sunny areas or for saltwater chlorine generator pools where minimizing SWG on-time is desired to have pH be more stable, then a higher CYA of up to 80 ppm is often used.