on the subject of the 11x15 cake and the many different number of slices you can get from the cake, how do you price out the cake? do you have just a set price for that particular cake?
I always charge by the serving. It doesn't matter the size/shape of the cake. All my cakes are torted & filled so it's still 4' high. I don't do a single layer cake with just frosting on it.
See similar topic on this thread: http://cakecentral.com/cake-decorating-ftopict-653932-.html
I price per cubic inch of cake. I have a base price for my cake. If the customer wants something costlier (costlier to make cake/filling/frosting, fondant, more complex decoration, gumpaste, etc) then the price goes up from there.