Is a 620 math SAT score decent?

I recently took a college board practice SAT test online and got a 620 math score(answered 39 correctly 4 wrong and skipped 11) and I’m wondering if that’s an okay score to hold for a real test which I’ll probably be taking after I get out of the Air Force( in four years). Should I buy some math books to aim for a higher score, or would a 620 be good to compete with?