The percentage is relevant to the original BPM. 0.8% will have a varying degree of BPM change for say, 120 bpm and 125 bpm track. I do agree that 0.7/08% can be used as a rough guide, however, you will still need to use your ears/headphones to fine tune it like you say. Hence, my non-understanding of how anyone can beatmatch without headphones.