There are no general rules. Certain days of the week may always be more hectic and therefore require more slack time than others. Leave that much free space between your appointments and Key Task time blocks. As you go through the day, it will make you feel better to have something worthwhile behind you. Most stress and fatigue is not caused by working, but by worrying about the things you didn't get done.
Success is simple.
Concentrate your efforts where they will produce the best and most lasting results - on your Key Tasks. Then, make sure you accomplish the task by selecting a specific block of time for completing it. Also try to work on your Key Tasks those hours of the day where you feel best and have the most energy. Do this before you leave your desk or work station. Little things like pulling out the file, writing the headline or an outline, laying out the tools or whatever will make it so much easier for you to tackle the job when you reach the time block allocated to your Key Task.