I had a client ask me this today. The simple answer: I don’t know. I wish I did. I wish there was a formula. But, alas, there is not.
There are some things to keep in mind. You probably want enough liability insurance to cover the value of your net worth. So, if you own a house with $200,000 in equity, you should probably have a policy that is at least $250,000/$500,000. If you have no assets, you may be okay with a minimum policy.
However, keep this in mind. Your uninsured motorist limit is the same as your liability limit. So, sometimes, your liability limit is not set to protect others, but rather to protect yourself. Let me put it this way: if 50% of all drivers are uninsured, there is a dang good chance that you will get involved in an accident with someone who has no insurance. If that happens, your uninsured motorist coverage protects you. A minimum policy does not provide as much protection as you may need.
With that said, I usually tell people to get the biggest policy that they can afford. You would rather have too much coverage than not enough.