When you read a book, never compare it with other books that you have read. Instead, give the book an independent rating by looking how the said book is doing on itself. Try to be open-minded and have no bias when you are rating any books.
There is no right or wrong way in rating a book. But having a book rating system helps you to record and convey your reading preferences in a more consistent way, and people can also depend on your book rating or reviews to help them make the decision whether a particular book is worth it to be read or not.