Early in Erik’s career when he was a young construction project manager, one of his managers told him to always pad his numbers. What he meant by that was that Erik should add cushion just in case something comes up. And Erik did that for several years until he went through the project management certification process and was taught that that is not the right way to do it.
Rather than padding your numbers just because, you should take a few moments to think about what risks are ahead of you. Anticipate what could go wrong and then you can adjust your numbers based on what you think could go wrong.
Instead of making something up without having a justifiable reason, think in this way: “So I purchased this $100 item but let’s say it doesn’t arrive on time. What effect could that have?” Now you have the rationale for why that $100 has increased. You’ve identified risks and mitigated it by adding more to your budget.
Do this, and your numbers and estimates will be a lot cleaner and you will be on target much more often than not.