My battery health has gone down 8 percent in the past 2 months.

I got this phone (iPhone 11) on 28th July 2023 when the battery health was 89% and today, 28th August 2023, the battery health is 81%. I have no idea what I should do. I charge overnight occasionally, I don't use my phone while charging and I don't let it go below 20%. I also keep it in room temperature.