I don't know if I'm qualified to answer your question but I got 80% in NIOS +2 in Oct 2016 exam I prepared for approx 3 months. I studied throughoutly in these months .

To get 90% is a bit difficult because checkers usually expect more in answer and checking is little bit tough. Its because 70–80% of students in NIOS just get passing marks and the evaluators have habit of giving Grace marks but not distinction.

But if you study hard then no one can stop you.

Also 70–80% in Nios is roughly equal to 85–90% in CBSE because of percentile score.

Nios top 20 percentile is about 62–63% and CBSE top 20 percentile is about 75–77% .