Project managers typically use one of the following approaches to calculate contingency buffers:
Option 1: Add a "safety increment" to every task.
Option 2: Place a buffer at the end of a set of project activities (such as a major milestone, a development phase, or an interim release) and/or as a separate task at the very end of the project schedule.
I do not recommend Option 1, simply padding the estimate for each task. It increases the planned duration and cost of the entire project by the expansion factor without differentiating among tasks. Protecting the tasks on the project’s critical path (the longest sequential path of essential activities that runs from initiation to delivery) against overruns must receive top priority. ...