Hi, you've come to the right place to look for answers, and I'd be happy to provide some insights!
It's normal for an iPhone battery to drop to 90% in the first 12 months of usage. A drop to 89% in 8 months is slightly abnormal but could make sense. I don't think you have a faulty battery but you should follow the following tips to prevent it from dropping fast.
- Makes sure you don't level your phone in extreme hot or cold environments. An example would be in a car in the summer. Heat can degrade the battery very fast.
- Try not to let your phone die. If it's about to die, power it off until you can charge it.
- Limit notification sounds and vibrations because these things eat at the day to day battery life (not long term).
Those are my main suggestions. Refer to this article for more: https://www.apple.com/batteries/maximizing-performance/.
I hope this helped, and remember that the battery is the fastest degrading part on a phone so after a couple of years it's normal to get it replaced.