The client has never run one of these. You have run four hundred

Publish the default timeline, and say it is a default

Arrival, drinks, seating, courses, speeches, turnaround, band, carriages. Put the usual durations next to each, say that any of it can move, and the client stops guessing. A venue that offers its own shape is not restricting them; it is giving them a draft to argue with, which is far easier than a blank page and is the thing they are least able to produce themselves.

The dependencies are what they cannot see

Speeches cannot start until plates are cleared. The band cannot sound-check while guests are seated. The room cannot be turned around with people in it. Each of these is obvious to the venue and invisible to the client, and each one is why a timeline that looked fine on a spreadsheet fails in the room. Naming three or four of them in writing prevents most of the renegotiation.

State the hard stops, and state them early

A licence end time, a noise curfew, a staffing cut-off, the hour the kitchen closes: these are not negotiable and the client will plan around them only if told. Discovered at the final meeting they look like the venue being difficult; stated in the first exchange they are simply the shape of the day. The assistant can state them because they do not vary per event.

Never confirm a bespoke timing in a chat window

A timeline interacts with staffing, kitchen capacity, other bookings and the licence, so an assistant that agrees to a nine-thirty first course has made a commitment nobody checked. It can describe what usually works, name the constraints, and pass a specific request to the coordinator. The useful answer here is the default plus the limits, not a yes.