I can realistically state that if you made that the pass mark, the pass rate would be down to about 5%! (Many of those who pass do so by the skin of their teeth.)I would imagine that 85 / 90% would be a better pass mark, saying that, shouldn't you know more like 98%+ of what you are doing?
(I won't say how we know - trade secret - but we do know roughly what mark each of our students is likely to get. It's one of the reasons we get the best pass rates in the country.)