Try this
- Start with the required arrival time and check current travel information appropriate to the journey.
- Add each person’s preparation needs, then state one departure time. Confirm whether the plan is firm or still based on an uncertain estimate.
A worked example
“To arrive at nine, we need to leave at eight twenty.”
An invented example to adapt, not a real exchange or a script you have to follow.
Before you leave it here
State the departure time plainly rather than hoping both people calculated the same buffer.