An office lunch and a meeting spread are not the same service. Say whether people eat at their desks, in a boardroom, or across a long event day.
The useful brief is short: how many people, what time the food should be ready, and which dietary needs are already known. A round number is enough to start. A late change of fifty people is not.
For an annual gathering, the meal follows the run of show. Service starts when the room is seated, not when the kitchen is ready.