I am rereading the book Essentialism by Greg McKeown, which I highly recommend to anyone who is wanting to gain some/more control in their life. One of the points he makes is to create buffers.
When trying to prepare for the unexpected, allowing yourself a buffer (or margin) gives you some flexibility and ability to absorb these occasions, especially when it comes to time and money.
McKeown recommends to add 50% time to activities. Think it's a 30 minute meeting-plan for 45 min. Got a project that you think will take a weekend, add an extra day or two. Same applies for money. I like to add at least 10% to forecasted expenses.
Of course, if you don't utilize this buffer zone, you come out on top with more resources to utilize or even relax.