I have a complicated ratings system.

0 to 2 points for plot and character development.

0 to 2 points for writing quality i.e. vocabulary, symbolism, etc.

0 to 2 points for how much I enjoyed it.

So, in theory that's 0 to 6 stars but of course I have to round to 1 to 5.

However, since I wouldn't normally finish a book which I rated 0 in any category almost all books are rated 3 to 5