set macbook air to 80% charge?

The best advice I can give for getting the most from your battery:

• Plug in when using the computer near available power.

• Use on battery when ac is unavailable.

• Don’t subject the computer and battery to extreme temperatures.

• Let the OS and charging circuits manage the charging.

• Try not to allow the battery to discharge deeply regularly before plugging in. Yes, it may happen from time to time, just don't make it a habit.

• Don't obsess over battery life and/or health.  

Here is Apple's guidance on the subject.

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support 

You can let macOS manage the battery charging process.

or

There are also a few apps that will let you manually keep the charging limit to 80% (or a value you can choose).

Google "app macbook 80%". The one I use is named after a way that pasta is cooked.

sgreifer wrote:

How should I optimize battery life on my new macbook air?

The built in option is under Settings -> Battery -> click on i to the right of Battery Health and this brings up another window with the Optimised Battery Charging setting.