I use a rating system similar to the grading systems used in schools. Instead of A+, A, A-, B+, etc., I assign numbers. I feel it allows for better delineation and scoring rather than a 5 point scale. I measure things in my job using this type of system and it makes more sense to me. These numbers more accurately represent how I feel about a book.
So without further ado:
100-90 would be in the A range
89-80 would be in the B range
79-70 would be in the C range
69-60 would be in the D range
59-0 would be a failure, so why assign a letter.
Honestly, I will not finish books I don't like for some reason. I don't see the point when I have so many other books I want to read. Therefore, 99% of the time I like all the books I read, but some not as much as others.
