I do not know if you are talking about the flight logger, or the Timetable in the website.
The Timetable in the website contains takeoff and landing times. If you send in a PIREP, then after the flight duration (from takeoff to landing) the departure time will be recalculated for the flight in the timetable. And it does not matter what was your block time (from startup to shutdown) which is important only for calculating your total flight time, but not the main timetable.
According to this, if you see, that a flight departs at say, 10:00 you should startup about 5...15 minutes before it, so around 09:45. It depends on the airport size, and amount of AI traffic. I usually sit into the cockpit about half an hour before the departure time. It is well enough for pre-flight checks, route creation, for boarding the passengers and for the airport to fill it with life: as the AI planes start moving. During the first 10-15 minutes many AI planes will leave for takeoff, and from the amount, I can plan how much time should I need for taxiing, to be on the hold short point at Departure Time.