The Best Guess table says your shock level, with a CYA of 40, is 15ppm, not 20.

I would suggest, if you are uncomfortable closing the cover with the FC at 15, that you wait until the water temp is around 60deg or lower, and then let the FC drop before you cover. Algae doesn't grow much in cold water.

Beyond that, I always use a mesh cover. Not this year, as I'm rebuilding the pool and there's no water in it. Also no liner and no walls!