When you begin to calendar items, take a look at everything you have to do and how long you think it will take to get each one done and then add “cushion” time to each item, this will ensure proper time management. I haven’t been able to come up with a rule of them for how much extra time should be given to each activity. It is dependent on you and the activity. You may have to explore in the beginning to figure out what works best for you based on how well you guestimated the initial time. If you actually do finish before the time you allotted, you have the gift of time to move on to the next thing. Put everything on your calendar. In time, you will become better at how much time you actually need for each item.
You want to also make sure to include buffer time for something that has a deadline. Say you have to submit a proposal to a prospective client by a certain date. In order to have it done, you determine approximately how much time you think it will take to create the proposal and then add a bit of time to that total. You then give yourself a deadline for completion that is 2 or 3 days before the actual ‘drop dead’ deadline. This means that if you have issues and unforeseen things that occur, you still will complete it by the actual deadline date because you have focused on proper time management. This works well when you have to rely on others to help you get something done. If you get it done early, all the better for you and what impression it will make with the prospective client.