How do you estimate food for a party?

[Marisa6826]

I'm planning on throwing a combination house warming/40th birthday party for Jonathan.

I scoped out Costco yesterday, and I think I can get the vast majority of the food there.

I'm planning:

Hotdogs and Hamburgers
Chicken Apple Sausage
Fried Chicken (not from Costco - still mulling over whether or not to get)
Potato/Macaroni Salads
Crudite
Salsa and Chips with Guacamole
Fruit Salad
Cookies
Ice Cream Cake (Carvel or Haagen Daz)
Beer (pony kegs or bottles)
Soda
Water
possibly a margarita station (since we NEVER use the smoothie maker he got as a birthday present three years ago ;) )

So how do you estimate food? It's a 3pm party, and it will probably be a late lunch/early dinner thing for most people coming.

I'm sending out about 40-50 invites, and the invites will apply to entire families, but we don't more than half will show.

-m