Estimate airport security and transfer time.
The deadline that matters is not departure time but gate closing, which is typically 25 minutes before a domestic departure and 45 before an international one. Working backwards from there, you add walking time to a gate that may be a genuine ten-minute hike, your security queue, and bag drop if you are checking anything — bag drop counters usually close earlier than the gate and the queue grows with each additional bag. Only then do you add the journey to the terminal. Building the estimate backwards from gate closing rather than forwards from departure is what stops the classic error of arriving on time for a flight you have already been offloaded from.
Time needed at the airport
Airport minutes = bag drop + security queue + walk to gate + gate close margin + buffer
Departure from home
Leave = airport minutes + journey to terminal
Because that is the moment you are actually offloaded. Departure time is when the aircraft pushes back, and boarding closes 25 to 45 minutes before it. Planning to departure time rather than gate closing builds a guaranteed shortfall of exactly that margin into every trip.
Yes, and it is often the tightest constraint. Bag drop counters typically close 45 to 60 minutes before departure — earlier than the gate — and the queue scales with the number of bags being tagged ahead of you. Travelling with cabin baggage only removes this entire stage.