National Merit Scholarships 2017 NY

I’m a New York junior who just took the PSAT, and I was wondering about how many can one GUESS you can afford to get wrong on this test in order to have an index of 221. Thanks!

@alexmozzerella My S had a 221 last year. He had 3 wrong on reading, 1 wrong on writing,(740) and 4 wrong on math.(730) Of course ever test will be scored differently.