I agree with ec52. When you assume you know everything, you tend to make mistakes. Consulting someone who is an expert in the area of interest can be a tremendous help with scheduling. The Subject Matter Expert will be able to give a realistic timeframe for the completion of the task, while also being aware of overworking others. It is also good to remember that when you first create a timeline, it will be inaccurate. Things happen and some tasks will finish earlier or later than expected. Buffering the tasks is still a viable option of making sure a project gets completed in reasonable time & within a projected timeline.