The story line is marked out of 10, purely because it is the most important. If the story isn't good then it's not a good book so you will not want to read it! It's as simple as that.
Well written: /5
A book perfectly written doesn't come around too often, but when it does it makes it that extra bit perfect so therefore we mark it out of 5 on its structure, spelling, punctuation and grammar. Picky, I know, but when it's right it's just so perfect!
Obviously you have to enjoy a book when you read it and if you don't then, umm, don't read it.
Just add them up, pretty self explanatory.
5 stars (18-20) Amazing. Everyone should read it and own a copy, if you don't then you must
4 stars (15-17) Still pretty awesome. There are only a few bits that aren't quite right
3 stars (12-14) Fairly average. A good read but there was no 'wow' factor
2 stars (8-11) Not great but still managed to finish it
1 star (5-7) Terrible. Boring, uninteresting and very nearly a book given up on
0 stars (0-4) My eyes, they burn