The 80% range means that 80% of the previous admits were within that range. So in your case 80% were within 550-670. Your 590 is right within that range so you are "in the 80% range" for your school.
Approximately 10% of the students were below 550 and 10% above 670... so your chances greatly diminish in that bottom 10% range or lower and greatly increase in that top 10% range and higher. You are right in the sweet spot so I would spend time on the rest of your application!
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