56 questions in MAT Dec'17. How much score can this lead to with 85%+ accuracy?

Well let's see.. First of all 56 attempts out of 200 questions is a bit on the lower side.

However your percentile depends upon the question paper itself whether it's of medium, easy or tough level..

If its tough you may get 80-90% or even more, and if it was of easy or medium level then I think you should get 60-70% or so...

However you can never be sure about how much percentile you are gonna get as it depends upon the other candidates and not just you...
So hope for the best...