Percentage of understood human DNA
The devil is in the details, and therefore we cannot just state that we have understood X% of the DNA. We know e.g. that 2% of the human DNA encode proteins. And for a good number of proteins we know what they do. So you might know that a particular codon AGT encodes a serine residue in a protein, which could have a catalytic activity. Or it has a structural activity. Or we are not sure what it does. Sometimes protein-coding and regulatory sequences overlap. So even if we first understood the protein-coding function, there was still the regulatory function to be discovered.

The upshot is that there are multiple levels of understanding, and big controversies of what they actually imply (see e.g. the ENCODE saga).