If you are scoring 90% because you are looking up 75% of the questions before you click on an answer, then you won't know what you've retained. 

Answer the mcqs without the book.  Mark the ones you felt you had to guess on, but got right.  The ones you got wrong will automatically be marked.  This will give you a much better idea how much of the material you are absorbing/able to apply.

You may find that you can still score 80-85% without the book, but judging the number you marked yourself will tell you if you have a firm grasp of the material, or if its in your brain (enough to guess correctly) but the edges are kinda blury.