My references to using DNA as a proxy for relative passage of time was referring to individual nucleotides. I see from other posts you were referring to functional genes. In that case, two species of roaches, hundreds of millions of years removed will be more similar than fish and people. But I still maintain my point about how 1 genetic difference will be revolutionary, while others will have little effect and will not cause speciation.
Similarity between species when comparing DNA goes by category of similarity.
Genes are the most similar, they have functional constraints, a hemoglobin gene cannot make a protein that doesn't bind oxygen and remain functional.
Proteins are even more similar because the DNA code is redundant, many of the differences between species in genes are “silent” mutations because they don't actually change what amino acid is being coded for.
Regulatory sequences can also show high evolutionary conservation between species.
That is why usually endogenous retroviral sequences are used, or pseudogenes, or long repeat sequences. They are more different than genes in the same comparison.
For example humans and chimps are about 98% similar in their genetic DNA, but only 94% similar over the entire genome.
That difference is magnified when comparing a more distantly related species like a mouse or rat.
The genetic DNA would be about 85% similar, but the genomic comparison would show a much greater divergence.