Time To Go – an alarm that shows how long your commute takes (apps.apple.com)

If I have a meeting at 11:00 and I have 40 minutes of conmute, I add an alarm at 10:10 that says "Meeting 11:00". So I have 10 minuter to prepare and then the time to travel.

Is it possible to add some pad time? How optimistic/pessimistic is it to calculate the travel time?