Most spa users do not like soaking in a high FC (regardless of CYA level) because during the soak the ammonia from their sweat and urine forms monochloramine immediately (CYA doesn't slow that down very much) and some other organics such as creatinine also get oxidized and these all smell (dichloramine and nitrogen trichloride especially). Urea is slower to oxidize but for a longer soak its oxidation by-products could also smell. So the recommendation is instead to add enough chlorine RIGHT AFTER one's soak such that they measure a small residual of 1-2 ppm FC just before their next soak. So yes, during a soak the FC will get towards 0 during part of the soak, but it will be for a short time and there will be a small amount of monochloramine (not more than 1-2 ppm) there to inhibit bacteria. Even if there wasn't, bacteria take 15-60 minutes to double in population even under ideal conditions so in practice the blast of high chlorine right after the soak kills them off and prevents biofilm formation. So the only risk with this approach is that one does not have the fast kill times to prevent person-to-person transmission during part of the soak, but in residential spas this is not normally an issue. One could not use this approach in commercial/public spas.

The key to having the spa be well-maintained is adding sufficient chlorine after a soak to oxidize all of the bather waste. A rough rule-of-thumb when there is no ozonator is that every person-hour of soaking in a hot (104ºF) spa requires 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of MPS in order to oxidize the bather waste. Of course, one should adjust their dosing such that they measure 1-2 ppm FC just before their next soak -- the rule-of-thumb is just that and not an absolute dosing amount so one should adjust accordingly.

For those who do want some protective FC level during their soak, they can still use this method but target a higher FC level for the start of their soak and yes one could have a somewhat higher CYA level to compensate for that. However, for most residential spas, one doesn't need to get much higher than 4 ppm FC or so. In a 350 gallon spa, one person-hour would be 7 ppm FC to oxidize bather waste, but the urea is slower to oxidize so in practice one doesn't need that full amount in order to maintain FC throughout a normal soak time. Again, one can simply target whatever FC they need to start a soak such that at the end of the soak they still measure some residual FC (a least 1 ppm). Most people don't do this because they would rather have a clean-smelling soak and understand the risk of person-to-person transmission in their own spa with (typically) their own family is low.