At what percentage of battery should I charge my MacBook Air?

Depends on how you have you Battery settings in System Preferences. If you have "Optimize battery charging" enabled and you use you Mac mostly on the charger, it could drop to below 80% before it starts to charge, unless you chose 'Charge now".

If you have Optimize charging disabled, it should start charging immediately until it reaches 100%.

You can leave the computer plugged in whenever needed or when convenient to do so. Leaving the computer plugged in will not damage the computer or the battery. The battery will not overcharge. Let the MacOS manage the computer's power/charging system. You do not need to micromanage the power system.