A few years ago I took a course in error correction. I could not remember if the actual data was 60% or 40%. Since you have a 70% figure, I suspect that the actual data is between 30% and 40%. Kind of amazing, isn't it! As I recall, there was one scheme upon another for checking the data at various levels. Some of the 60 or 70 percent may have been formatting for addressing, but a huge amount was redundancy.