Thanks for posting your questions and results. Please keep posting your results.
I am also going thru this same thing. We just converted to SJ55 and fired it up on Monday evening. Ran it at 75% for 9 hours and it only brought the FC up 1ppm overnight. I bumped it to 100% last night and it ran for 9 hours. It brought the FC up 2ppm. This morning, I bumped the FC with liquid to 7 and I turned the pump on to run all day while I'm at work and set it to run at 50% to see what happens during the daylight/sun. Tonight I will do the OCLT to see if I have anything going on and need to slam. Pool has been open and balanced with no problems for a month, so I don't think I will be seeing FC loss overnight. We did have guests who applied sunscreen several times while swimming Monday afternoon, so I was thinking maybe my FC loss was possibly fighting that. My CYA was at 50 and I added stabilizer last night that should bring it up to 60. Once it levels off, I will add more to ultimately bring the CYA up to 70. After dosing with liquid chlorine for 6 years and keeping my CYA at 40, it is hard for me to raise that CYA to 70!!!!
I'm hoping it's just a matter of playing with this thing to figure it out. I normally run my pump at night, just because I think it's easier on the equipment to run when it's cooler, but does it make more sense to run it during the day to keep the steady flow of chlorine generating since the CYA level is higher?