Using the Fibonacci sequence will help you to have borders that are pleasingly well-balanced to the eye. Here are a few websites that explain:
http://thunderbayquilters.org/?page_id=381 Scroll down to where it says "Border Widths" It gives a very good explanation of the Fibonacci sequence, also known as the golden rule.
https://comestitchwithme.com/2014/09...h-for-a-quilt/