I did what Earlene said that she did and flipped my cake pans upside down and marked out 1"x2" to see how many servings actually come out of the cakes. It was right on Earlene's numbers, of course, so I use her chart. I also tell brides that I have no control over how the person cutting the cake cuts it, but that if anyone cuts it differently than I tell them to they'll probably cut the pieces smaller. That means they won't run out of cake servings, anyway. I heard that one coordinator at a place I take cakes all the time told one of my brides that she makes sure to cut the pieces "really small so that you'll have a lot of cake left over!"
I'll be talking to her the next time I deliver a cake out there!