What’s the best charging routine and battery percentage for the iPhone?

Here’s an article that may help answer this: iPhone Battery and Performance - Apple Support

You can charge the iPhone battery to 100% any time you like, and it will not negatively affect it. Just make sure to turn on Optimized Battery Charging in Settings > Battery > Battery Health, as this is what the iPhone uses to manage its battery and make sure it doesn’t “overcharge” so to speak. But while there’s no perfect daily routine, I can tell you mine: I take my iPhone (13 mini) off the charger by 7:30 AM, use it normally throughout the day, and put it on the charger for the night at around 11 PM. Now, if I use my car at all during that day, I always stick my iPhone to the MagSafe charger car mount that I have on my dashboard, so that it can charge while I drive and play music over CarPlay. And any days that I use it quite heavily, due to the 13 mini having a smaller battery, I may charge it on my Anker Powerwave wireless charging pad during dinner if it’s at or below 20% battery. Alternatively, you can use Low Power Mode; I just prefer not to because I’m a bit of a power user/gamer and Low Power Mode does affect the graphics performance.

Hope this helps.