I already have a price per serving for 3-d cakes, but my problem is how many servings do I have left after you go hacking away...so I can chargremy "per serving price". I try to estimate in my head how many servings i have chopped off and thrown away and deduct those. Or should i go by the original size of the cakes before cutting away I could never really figure this one out. What do you do? Thanks!

You go by the size that the final cake feeds, but you charge more per serving for the greater difficulty and the lost cake.

To determine the number of servings, you have to determine the volume of cake compared to the slices. This is hard because every 3D cake is different. For me I look at it from the perspective of if you were at the party how would you cut the cake.

Let's take a simple 3D cake like a corsette cake. I'd probably use the base rectangle shape and deduct 3 to 4 servings for carving in the waste. Add 2 servings for the bust and call it done.

But if you're talking about a 3D dinosaur, now you have bigger issues. There are hills and valleys to the cake. Look at your cake design and plan a cutting chart. The flat pieces would be one calculation of servings. The taller parts you may cut tall slices and then cut them in 1/2 so that may yield 2 servings per cut. Does that make sense?

There is no easy way to do it.
Hope that helps a little.

I weigh it and calculate 100 g/portion which is a standard portion here. (The cakes are sold by weight - I do not sell them by weight but in this case it actually helps). And charge much more per serving than for a simple cake.