This we actually can measure - speech recognition's comprehension rate is a big problem for its implementation.
So we know 80% accuracy is not good enough to produce cogent sentences (1 miss in every 5 words). And we know 1 miss in every 10 still doesn't produce something we'd think of as accurate.
So I'd say you need about 90%+ comprehension to actually comprehend something. But that's leaving a lot of room for error anyway - how much stuff have you read and then thought "wait, what was the modifier to that specific phrase?"
So we know 80% accuracy is not good enough to produce cogent sentences (1 miss in every 5 words). And we know 1 miss in every 10 still doesn't produce something we'd think of as accurate.
So I'd say you need about 90%+ comprehension to actually comprehend something. But that's leaving a lot of room for error anyway - how much stuff have you read and then thought "wait, what was the modifier to that specific phrase?"