There is not a specific amount that you're allowed to go over the speed limit. However, if you consistently go 8-10mph over or under the speed limit, it's unlikely that you're going to be successful. Your speed management is directly linked to your scanning pattern. You should be checking your instrument panel as part of your scanning pattern every 8-12 seconds when driving. If you aren't adjusting your speed every few moments, it tells the examiner that you don't have your scanning pattern in place.