I don't see why having a rating system out of five stars necessarily means it has to be 1 star = 20%
Why couldn't you have a more exponentially graded system that gets tougher to get the 5th star.
0 stars = 0
1 star = 1-35
2 stars = 35-65
3 stars = 65-85
4 stars = 85-95
5 stars = 95-100
You could even shift the scale if you wanted to, or fit it towards a bell curve so its harder to get a 1 or a 5, than it is to get a 3.
All I mean is that if you predetermined your grading and made it transparent to your users - values don't have to directly correlate to 4 stars = 8/10