You're going to have to work out the size of your expected claims, and the likely frequency of claims to get an idea of your exposure. That should give you an idea of your risk. Knowing your risk gives you some idea of of the coverage you should have. Now shop for prices. You simply can not assign a percentage of income.
If this is auto liability, I'm sure your state is going to have input on the amount of insurance you are going to have. Again, % of income is not especially relevant.