So as we debate how many servings are in this or that size tier, we lose sight of the fact that we have no control over how people cut the cake. We can talk until we're blue in the face, but if we don't personally cut the cake every serving chart in existence is only an estimate.
I've been to events where the person cutting the cake cuts it much thinner and wider than it "should" be cut in order to make it fit the dessert plates better visually. This looks nice on the plates, but it also means that the servings are much smaller than what I'm estimating they'll be. So you'll get more pieces out of the cake, but each piece will contain less cake than the standard serving.
I estimate 80% of the number of guests for the number of cake servings. Some people around here tell brides that they need 110% of the number of guests for the number of cake servings. However, the venue staff tells me that they always have entire tiers of cake left over when those people bring the cake. Looking at the size of the pieces, yeah, that makes sense. In the first place, not everyone has cake, and if the pieces are cut so thin you'll get more pieces out of it than we estimate.
It's worth asking the people at venues how they cut the cake. It can make a BIG difference in the amount of cake a bride needs. If you know that a venue cuts smaller pieces you can let her know that, and she can decide whether she wants to eat leftover wedding cake for the week after the reception or not.
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com