I jotted down a few disorganized thoughts in this thread: http://www.cheftalk.com/forums/profe...ngs-chart.html
Generally, the best answer is to consult closely with your client and try and get the best sense of the group you can. Then using both your heads to plan, create and serve the best meal possible. Introducing the idea of "better too much than too little," is pretty easy when you do it early. Especially if you put it in the form of a question and let her say it.
When I still did it, I tried to create a situation where the client could come downstairs dressed, and look into the kitchen and feel comfortable chatting with me and my crew while I made her a drink That meant me being comfortable about everything, including quantities.
In terms of portioning it meant knowing I wasn't going to run out. That's really the nubbin isn't it? Knowing you're going to have a little too much, but not an obscene amount. I'll tell you something though. It's easier to hide "way too much" -- even if it means you have to rebate a little -- than even a little too little. And even if you can't hide it, it's a lot easier to say, "they always ate two ducks each before," than it is to say, "your family eat like pigs, don't they?"
Having someone on staff that can pack leftovers attractively is a VGT. Once a few aluminum foil swans go out the door -- you usually don't have to worry about leftovers.