I just did a search for this and really didn't find the answer I was looking for. I do understand how to construct a topsy turvy cake, just not how to configure the number of servings for it. I'm looking to make a 3 tiered one for a client that serves 40 in a round shape but I'm not sure how to configure the number of servings nor what size cakes I should start with. Anybody know where to start?

I calculate the servings same as a regular cake of the same size. The only difference is I bake all the layers a little higher (5" where I've called it 4") to make up for the carving at the base.

So it's safe to bake a 4",6",8" round serving 40, only make it taller and then carve. I won't loose any servings?

My method is to calculate it based on the diameter of the bottom of the cake.
So for example if the bottom tier of my topsy turvy was 10 inches at the top and 9 inches at the bottom after carving inward, I would calculate the servings as I would my typical 9" round cake.
I figure with carving out the center for leveling, etc... it should be about right or at least in the client's favor.

Hope that helps!