How to stop charging Apple Watch ultra at 80% witch Optimized charging on

I want to save battery life and to program Apple Watch Ultra to stop charging at 80%. Optimized charging is on, but I always get full charge. I have never seen the open circle. I always exceed the 80% charge when I try to stop charging myself because it’s difficult for me to constantly keep an eye on the charging process. How can I make sure that my watch stops charging ar 80%? What do I do wrong. This gives me stress.