When estimating your time in planning your calendar, it is important that you overestimate the time it will take to complete a task. That is because we are conditioned to think things will take a lot shorter than they will take. So we end up underestimating our time and then finding we run out of time at the end of the week.
A good rule of thumb is to add 25% to your estimate. This will help you create gaps in your calendar. So if you can stuck at an appointment you know you will be ok because you have allowed extra time in your schedule.
You can also find that you might accomplish tasks quicker too. This can create extra time in your schedule. You may be tempted to fill this up with other things but you are better off leave it as it is. This is because plans often do change and having extra time can be of a benefit later on.
By doing this, you can make sure you keep to your schedule and complete all your study goals that you have planned.