I don't believe it's possible to measure it in that way. You might get 80% on one, 60% on another, and still be in good shape for the real thing. Remember that it is more about a relative scale, how well others are doing on the same exam. If you know you're score constantly is above average on Book 6, then you might feel more at ease. Make sure you know how to "take" the questions quickly and effectively, and read or do as many exams as you can possible muster and you'll probably be fine. If you experience a time crunch towards the end, you may even start to read questions and the corresponding answers without "having a go" at them, simply to learn the style of how the questions are worded and to learn what they're after. Every little thing helps.