I have a customer who wants a 4 tier topsy turvy cake to serve 75-100 (I know, big range, so I'm going with the 100). What pan sizes would you suggest? Usually I do 3 to 4 inches difference per tier with my topsy turvys, and the ways I have figured it are yielding too much or too little. What sizes would you use?

Here is a sample of what these sizes will serve.
6" - 12
8" - 24
10" - 38
12" - 56

HTH!

Also, These were all for Round cakes. What shape are you doing?

I am making one for a July 1st wedding. The following is the sizes of layers I am using on each tier.

Tier #1 - 8, 7, 6
Tier #2 - 10, 9, 8
Tier #3 - 12, 11, 10
Tier #4 - 14, 13, 12

I have it calculated that I will have around 134 servings using the smallest layer from each tier for the calculations.

HTH

Myra

This topsy turvy tutorial serves around what your looking for:
http://www.wickedgoodies.net/2012/08/how-to-make-a-topsy-turvy-cake/