I think for general listening, 70% and above is good. 100% is not realistic since many publishers release music that's badly clipping (so it sounds over-bassed and louder).
If you have a good audio system, and by that I mean high-end speakers and amplifier, or high-end headphones, then you'll hear a difference between 50% and 100%. But if you listen mostly in your car, or with basic earbuds, or computer speakers, you won't hear much of a difference.
So just listen to the song and decide whether it's acceptable quality to YOUR ears