I have to preface this by saying I very rarely give out 5's. A book has to be pretty stupendous on all levels to achieve that perfect score. When deciding how to rate a book I think about the writing, the story, the characters, emotional appeal, all kinds of things really.
For example, a beautifully written book that doesn't engage me won't get a high score, although the writing will probably save it from being on the bottom of the scale.
Also a fun, brain candy book that keeps me turning the pages can rank high but not a 5.
Sad is good! I'm a sucker for sad movies and sad books! But seriously, a book that can make me feel something gets extra marks, or one that makes me think.
So my way may not be mathematical but hey, ratings are all subjective anyway!
The rating guide I follow:
5 - Brilliant! One of the best books I've ever read! It spoke to me! It moved me! WOW!
4.5 - Terrific! A page turner- I couldn't put it down! Well-written, excellent for the genre.
4 - Very Good - Makes me want to read more by this author.
3.5 - Good - an enjoyable book, worth reading.
3 - OK - Enjoyed most of it but something was missing or disappointing.
2.5 - Mediocre - Neutral or mixed feelings.
2 - Disappointing - Finished but overall didn't enjoy it. Wouldn't recommend.
1 - Painful - Barely maintained my interest; a struggle to finish.
0 - Awful! One of the worst books I've ever had the misfortune to finish! Don't waste your time!
DNF (Did Not Finish)- Completely lost interest for one reason or another.