60 is the general baseline here at TFP and certainly about the warmest you want it, but if you can wait longer and let the water temp fall more that's even better. Some owners don't close until their water temp hits 50 or lower, weather conditions permitting of course.. The golden rule - close late open early. The colder the water the better.
Hey lil !! The problem with the weather for all of us is that the water temps are prime for closing on a particular week. And don’t you know, the unexpected heat wave shows up exactly 9 days later. So then you’re too warm again. We had a winter that was so out of whack a couple of years ago that we had 60/70 degree days in Jan and Feb and March/April were borderline freezing. I was gifted a swamp through none of my own doing. So the best you can protect yourself is to just wait as long as you can in the fall and get a good early jump on it in the spring. The water is real easy to maintain early/late and you can go a week or weeks without needing to add FC.
Plus, it’s always so much nicer to see the clear water instead of the cover. Even if nodody dares to think of swimming.