I have an auto cover and a SWCG and it does take a little bit of tweaking throughout the year to find the right percentage. In the heat of summer and when the cover is open most of the day and everyone is swimming, we have to keep the percentage higher. My cell does not produce chlorine when the cover is closed (automation with the cover turns it off) so it takes a little more thinking to find the right average of open, closed, swimmers, UV etc., but you will find the different variables that you have to worry about to find the right percentage.
This time of year, when there are less swimmers in the pool, and it is covered more, and the UV is not as extreme, I find it needs to be turned down. If my cell was on when my cover was closed, and there was no UV, and no swimmers, the chlorine would continue to climb, as you have seen in your pool.
According to Pool math, your T-15 Pro will produce 2.4ppm of chlorine each day, assuming you dial it back to 25% and run it 24 hours per day. If you keep the cover closed for two days, you will add 4.8ppm, and nothing to burn it off. This time of year, you are probably going to lose 2-3ppm per day if you keep the cover open, so there is a little math to do to find the right percentage to run it for the rest of the year.
