I know I'm late with this answer, but the idea is to see if there is chlorine loss when the sun isn't out, so it doesn't matter whether you shock , then take the night's reading, or don't shock, take the night's reading, and then shock it AFTER the morning reading. (Won't tell you anything if you shock it between the night and morning tests). However, with the amount of rain you've had, and the high stabilizer, I would go ahead and shock it as soon as possible. I run my pool with CYA of 90-100, and if I left my Cl at 4 or lower, it wouldn't take more than a day or so before it would start clouding up.

Janet