The approach to this very much depends on how you approach learning.
I found it good to read and then re-write the notes in my own words and then hit the bristol question bank. Theres been much discussion over this on here but as an approximate guide for every 5 hours reading and re-writing notes I would spend 2 hours bristol bashing. Sunk in much better that way throughout. Then when you go to sit the exam make sure you lock yourself in your hotel room with a large pot of coffee and spend about 5 hours on bristol for every 2 hour of reading your notes.
Biggest tip I could give you is to know when to stop and put the books away for the day. I often got suckered into doing 50 or so questions on Bristol til well after midnight having hit the books for hours and hours before and having been in a class room all day. This was more often than not counter productive because my brain was slowly being turned to mush and needed sleep.
To answer your question honestly.... My groundschool always advised its students to be achieving at least 85-90% in mock exams (not the bristol database) before sitting the actual paper because they had found that on average people lose between 5 and 10% when it comes to the actual day due to stress etc etc. For whatever reason I found I lost upto 5% but still got well over 90% in all subjects (except Gen Nav which was a bastard!) If you spend long enough on Bristol to be honest you'll more than likely pass the exam....which is ok when it comes to Comms but PoF is very very relevant when it comes to flying so better off learning that at thoroughly as possible.
Hope this helps but give me a shout if you have any questions you need a hand with.