The recommendations assume good circulation, non-pitted pool surfaces, and elimination of nascent algae prior to maintenance levels. If one has a pool that has existing black algae growing in deeper pits or yellow/mustard algae behind light niches or has areas of poor water circulation, then a higher FC/CYA ratio would be needed to prevent algae growth. That can be accomplished through a higher FC or a lower CYA, but doing the latter would usually require the SWG to be on longer in a pool exposed to sunlight and would likely have a faster pH rise and more acid needed as a result.
Of course, everyone's situation is unique so while most pools operate well at the recommended levels, one can adjust accordingly. Susa, your post above is the first one I see of you mentioning the black algae problem or your changing the CYA level down to 20 ppm. At this time of year, the sunlight is less intense and not all days are sunny so having a lower CYA level can be OK, but during the middle of the summer let us know if you are able to keep a reasonable on-time for the SWG and whether your acid usage changes.
