What the set-up changes in the cost
Three things, and none of them is the menu. Staff, which is the big line and depends on the walking. Set-up and break-down time, which depends on how many elements there are. And hired equipment, which depends on the type of table and the number of pieces per cover.
The practical consequence is that the price per cover is not a number belonging to the venue: it is a number belonging to the set-up. Offering the same price for rounds and for long tables means one of the two is wrongly costed.
Rounds versus long tables
Rounds of ten are the most efficient banqueting set-up there is to serve: one waiter covers a whole table from a single lap around it, and plating out is symmetrical. That is why they took over, not the look of them.
Long tables are more striking and more expensive to serve. Service happens from both sides, two people have to be synchronised per section, and the total walking distance is longer. For the same number of covers, the usual answer is one extra person per hundred.
Standing receptions: fewer chairs, more staff
A standing reception looks cheap because it carries no linen and no full cutlery, and it works out dearer than people reckon. The food circulates instead of sitting still, so the staff never stop: a banquet has peaks and troughs; a reception is one two-hour peak.
And the number of people depends less on the guests than on the service points. A reception for 200 in one space with two bars needs fewer staff than one for 150 spread across garden, terrace and function room.
What a tight capacity costs
It is the most invisible of the lot and the one that eats the most margin. When the room is tight, the service gangway drops from ninety to sixty centimetres and waiters stop being able to pass each other: every trip takes longer, service runs on, and staff have to be added to hit the same timings.
In numbers, a tight set-up pushes the cost of serving up by 8 % to 15 % against the same number of covers in a generous room. It is money lost on the event you sold hardest, which is why the sums never quite add up.
Getting to the price with the plan in front of you
With three figures the plan gives you directly: how many covers fit with a clear gangway, what type of table was used, and how far it is from the pantry to the furthest table. Those three give you the staffing, and the staffing is half the floor cost.
The advantage of doing it this way is not the precision: it is that the price can be explained. A client who asks why long tables cost more gets a concrete answer instead of «they are more work».