I've had my pool FC up at or above FC=25 (CYA=60) all week and still get overnight loss. When it was at 25, the next morning I measured 23.5. Then I brought it to 30 and the next morning it was down to 28. Yesterday I had it at 35 (by accident) and this morning it is 32. Seems like the more I add, the more it drops.
There are no indications of algae and the pool is sparkling clear. I only decided to shock when we got a ton of rain and I suspect fertilizer may have gotten in the pool from the lawn guy, and my FC dropped to 1.
I'm getting a lot of pressure from wife and kids wanting to swim. So I'm thinking of letting it drop to 10 and just leaving it there until November and shocking it again then if I need to.