A buffer should protect a known kind of uncertainty
A planning buffer is useful when the exact outcome varies within a reasonable range. It is less useful when a known step has simply been omitted. Known work belongs in the baseline; uncertainty belongs in the buffer.
For example, if cleanup always takes 20 minutes, include 20 minutes. If access to the work area varies from immediate to 30 minutes, that variation may justify a buffer or a separate waiting allowance.
Where to put the buffer
A task-level buffer is easy to understand for one job. A project-level contingency can be more efficient when many independent tasks are unlikely to all run late at once. A schedule can also hold explicit management reserve that is not automatically assigned to every task.
Whatever method you use, label the buffer. Hidden padding makes later learning difficult because nobody knows whether the baseline improved or whether the contingency was consumed.
Avoid false precision
A calculated answer such as 17.43 hours can look scientific even when the inputs are rough guesses. Round the result to a level that matches the uncertainty. For a loosely defined home project, “about two to three days” may be more honest than a decimal-hour result.
As experience accumulates, historical data can justify tighter ranges. The aim is not to make every estimate exact; it is to make decisions with an appropriate view of uncertainty.