In general, you would report in the state where the work was performed (Maine) regardless of where the customers are located. In your case, if you work from Maine, you can report the income to Maine.
However, each state has its own laws that you would have to apply in certain situations. For example. as you mentioned, the location of your business headquarters is a factor for a few states (including New York and Massachusetts) but since yours is in Florida you are okay.
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