Estimated number of servings for "walk-through" reception?? HELP!

I just posted this on my local Utah ICES Chapter page but I thought I'd get some advice on CC as well!

I'm relatively new to Utah but I'm REALLY new to the idea of a walk-through reception where there's no assigned seating and the couple invites everyone (everyone in their churches, everyone in their neighborhood, all coworkers, etc). Some of my brides have told me they're sending out over 500 invites with no real "RSVP" since it's not a "sit-down" dinner. I've delivered wedding cakes to venues and have seen lines of guests going out of the building and wondered what the heck was going on. I've learned it's a line to shake the bride & groom's hands! How interesting! Anyway, I haven't been to many weddings like this as a guest so I don't really know how to estimate number of servings for this phenomenon.

Let's imagine they're expecting 300 guests at the "walk through" reception (no sit down dinner). How many servings would you suggest?

What do you suggest when it's a walk through PLUS added side desserts (like brownies, cookies, etc)?

I'd like to expand the question to sit down receptions as well. If they are expecting 200 guests to be seated and eating, how many servings do you suggest they order? I've been getting a lot of people saying "Not everyone will stick around that long" or "Not everyone eats cake" and I totally understand that but I'm never sure what to suggest. I always suggest cutting the cake right after the couple is done eating to make sure the cake does get eaten but I'd love your thoughts on answers to those questions!

Thanks everyone!

Jennifer

Salt Cake City
 

Delicious Desserts, thank you for the number. So generally 75% for buffets? Sounds like a good number to me.

Baking Irene, I totally agree. They're going to feel bad if there's not enough cake! I also agree that cake for breakfast, lunch, and dinner is always a good idea, especially after the stress of a wedding :)

The walk through is just an interesting phenomenon because they don't necessarily take RSVPs so they could have all 500 show up or they could have 50. It's bizarre to me. I always ask what the budget is and just say we'll work with that. 

Thanks, you two :)
Jennifer