db.php & db-config.php | after install what are the most secure file permisions I can apply?

Hi WPMUDev,

I hope your day continues to be well!

I have a small Q about the post-install environment for Multi-DB.

My Question is:

After installation, what are the most secure file permisions I can apply to the db.php and db-config.php files on an ongoing basis?

(I think my server default is 644)

eg. Can I set them to 640?

Or should they be left as default?

Regards,
Max