I am having trouble coming up with cake sizes. The cake is to serve 30 people. The bride wants 2 or 3 tiers and wants them square. I was looking at a serving size chart and to only serve 30 people, it seems to me 3 tiers would be pretty small and funny looking. She wants to put flowers on the top and I'm thinking they would look better on 3 tiers and for only 30 people, a tier cake would look better. She wants 2 different kinds of cake so I have to do at least 2 tiers but I'm thinking maybe I should do one tier a dummy. What size square cakes would I make for 2 and 3 tiers without making too many extra servings. (She said she wants it to serve 30 but they think 20 to 24 people will show up.) They are not saving the top, they are serving the whole cake. Thanks for any help and/or opinions. 
I think you might just have to sit the bride down and explain that there's no way for you to make a 2 tier cake that will only serve 30 people. If she wants a tiered cake, she is going to have to pay for it.
A 6" square feeds 18 and an 8" square feeds 32. So that's 50 slices between the two. Of course, these are wedding sized slices. So you could always explain that they could cut the slices bigger. But of course, they still have to pay for the 50 slices.
Even if you use a 6" dummy, she's still going to have to pay you to decorate the dummy. She might as well serve a real cake for as little as that's going to be.
You can do a "mini" cake...2 inch, 4 inch and 6 inch to give you enough servings yet the tiered look. I did this for an anniversary party recently and it was a hit. If she wants a bigger cake you could do a 4 inch, 6 inch and then make the bottom tier 8 inches...too much cake but a bit bigger.
You know .... I really just get sick of brides who try to dictate "I want a 5 tier cake to serve 20 people". Well guess what, chic? It ain't happenin'! 
And I've no problem telling them so.
I understand that 7 times out of 10, they really and sincerely don't understand it, but give me a freakin' break .... she's never been to a wedding and has never seen how freakin' BIG a multi-tiered cake is and how many people it's feeding at that event???
Ignorance I can deal with ... they just need educated. Stupidity makes my a$$ tired.
The only thing I think to watch out for about doing the exact servings and doing the mini is your minimum price point--be sure it's high enough.
Of course a 7x4 is a viable option for 30 servings too.
This cakulator will help you determining different sizes/servings.
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