Door-to-door is bigger than moving time
A travel estimate can include preparation, walking to the vehicle or station, parking, check-in, security where applicable, transfers, waiting, baggage and the final trip to the destination. Which pieces matter depends on the journey.
A mapping or transport service should be the source for current route and timetable information. A general time planner can help add the surrounding steps and a buffer around that route estimate.
Match the buffer to the consequence
Missing a casual arrival and missing a booked departure have different consequences. More consequential events justify more contingency. Weather, congestion, border formalities, major events and accessibility needs can also change the required margin.
Avoid scheduling risky rushing. A buffer is there to absorb uncertainty, not to encourage speeding or ignoring safety requirements.
Keep transfer assumptions visible
If an itinerary has several legs, record the minimum transfer or connection requirements from the transport provider and then add personal needs. Do not rely on a generic connection time when a carrier, terminal or ticket rule defines the actual requirement.