If you make your tiers 6" tall instead of 4" tall do you count in more servings? what size would the servings be cut? is there a chart?
Well, if you're doing (2) 3" high cakes stacked on top of each other then I would take the serving chart you use for 4" cakes and multiply by .75 to get the number of servings for a 3" high cake.
No, it's the same number of servings because the surface area is the same as a 2-layer, 4" cake. The slice of cake is just taller. The only way to get more servings is to slice them only about 1/2" wide (instead of the standard 1" wide). And guests really WOULD talk about how freakin' paper thin those pieces of cake were.
When you do a 6" tall cake, be sure you charge more per serving than you would for a 4" tall cake. They are getting 50% more cake per serving (not to be confused with being the same as more servings), so the price per serving should be 50% more ($4.50/serving as compared to a $3.00/serving).
Also bear in mind that most dessert plates are 6" or just 7" in width. So a 6" tall cake, plus filling and icing, could result in a piece of cake that plops over the edge of the cake plate ... not very attractive. So just confirm with the client that the plates they use are big enough.