How do you agree realistic targets and why this is important?

How do you agree realistic targets and why this is important?
Let's say you have a project and a deadline. You need to make sure that deadline is fair and be honest about how much work you can handle. When speaking to a senior employee you will gain respect by being straight about how much you can do. If you set too easy a target for yourself you won't be pushed to improve yourself and it will be clear to your senior employee or your employer. If you make it impossible to reach they will be unimpressed by your inability to work out how long you need and missing the deadline. Setting targets is part of planning so the more accurately you can stick a plan usually the more likely you are to succeed.