Usually my wedding cake layers are 4" tall with filling between the layers. The bride brought a pict. of a cake that she wants that has the middle tier 6" high. 1st: the rule is for every 4" you need a cake board, correct? How would I do this. 2nd: How do you figure servings? What a 2 layer would serve then 1/2 of that for the next layer? 3rd: Usual serving size is 1 x 2 x 4. would you do the same for this cake?

TIA for your help.

You could use a cake board in the middle at 3" to serve as two separate cakes with slightly shorter servings of 1 x 2 x 3 (6 cu. in.) which would yield double the normal servings shown on your chart.

Yes, do exactly what CWR said. Not only will your cake be sturdier but it is easier to get all the servings and the slice will actually fit on a plate!