This is the first I've heard of that UV theory. It's entirely possible. The lower FC explanation is true. You guys (or one of you) is way overthinking this. I recognize the trait because I was the exact same way. It helps me learn and retain when I understand the why. So I can't fault you for that. But what I eventually came to terms with is the guidelines that TFP teaches are tried and true over a
large number of years and a
huge number of pools. They work. I kinda gave up on the why of it, for the most part.
The last I heard, Fiesta is running his SWG. If that's still true, he should use the SWG portion of the chart. The end. I also advised him to use at least 5 as his FC target while he sorts out his CYA and SWG and FC. Which is what he just said he's doing. I'm not sure what you're trying to reinvent, or why. It doesn't need to be any more complicated. He is working toward running an SWG pool with the correct CYA and FC, according to the SWG chart. There's no reason to postpone that. Let him get there.
[Yah, @mknauss, I hear you chuckling. Go ahead, I deserve it. You deserve to. Revenge is sweet!]
I like to run my SWG during the day. That's when the most loss occurs, so I like the idea of it being "replenished" every hour or so. That keeps the FC rollercoaster nice and flat. By running at night, you build up all night, without loss, then start the day with the full amount, and it declines all day until the next night's replenish. Bigger rollercoaster. I prefer not to do that. But I run a solar heater all day, so have to run the pump all day anyway, so there is no cost savings for me to run at night. If there is for Fiesta, that's a good enough reason to run at night.
That said, I'll guess 90% of pools get dosed once a week, have a huge spike of FC on Monday and end up with FC 0 by Sunday, or sooner. So compared to that, SWG dosing at night and still maintaining close to target by the end of the next day is more than fine.
Also, you don't need to think in terms of "algae deactivation," whatever that is. If your FC never drops below minimum, there is no algae in the pool. It's not something that is lurking, and needs to be tamed all day. It's not there. If there
is algae in the pool, then the target and minimum you're using is too low, and you would bump them a notch or two.
And lastly (hopefully), if the SWG is set and maintaining FC 5, it doesn't matter if his CYA is low for a while longer. The CYA is not part of the sanitation process. It's only there to help maintain FC. As long as FC is 5 or higher all the time, then the chlorine is available and able to do its job. If the CYA is low, that might mean that he's burning more FC due to UV than he needs to, but again, the FC is still there protecting the pool because the SWG is maintaining it. As his CYA comes into range, he may find that he can dial back the SWG to still maintain 5, because now less is being lost to UV. So again, overthinking. The goal is to maintain the 5, he'll eventually figure out how best to do that with both CYA and SWG. Until then, as long as it's 5, a little loss is only hurting the pocket book, not the pool.