I rate on whole marks, not half marks like a lot of blogs do. I used to rate on half marks (and might go back to it soon) but found it was a little easier with the whole marks.
I rate the books on story, characters, writer's ability and cover art. Like recently I read a book that I couldn't force myself to finish reading because I was so utterly confused about the direction of the story and what it was about. I would normally have given it only one butterfly mark, but since I enjoyed the characters and the cover I gave it two butterfly marks.