I have 3 different serving charts which I use based on the event and the calculation is based on differnt serving size. Like a wedding event which normally has lots of other food, one serving size is very small like 1x2x4, then there is an event like birthday which most likely serves slightly bigger size like 1x3x4. The link previously mentioned is very useful because every tier is not always 4 inches high. I know some of us are used to the big cake portion like Cheesecake Factory!
Isn't Cheesecake Factory charging much much more for their "serving?" and most people split those because they are so big, hence, two servings.
A 2-layer cake (whether for a wedding or birthday) is typically 4" inches high, therefore, 1*2*4. A party cake (or sheet) is 2" high, therefore 2*2*2. The are both the exact same amount of cake.
I can't imagine ever eating more than that in one sitting. It's just so unhealthy and sounds absolutely too sweet for words to described kindly.
I use and app called"cake slicer" http://cakeslicer.bakingit.com. You can pick your portion sizes with this app which I find useful as not everyone wants the standard size.