I'm making my sister's wedding cake. It's round 6" on a 8" on a 10" on top of a 12". But the layers will be hair bit shorter then 2" each. I did a quick bake and found its the equivalent of 7 cake mixes. So pretty heavy cake. 

So question is how do I calculate how much frosting this will take?

I keep reading how ppl use foam board. Are they using it between the layers or as the bottom board?

I checked out 1/2" fb at HL and had a hard time accepting that could hold up such a large cake.

Use masonite board from Home Depot (think dark brown peg board with no holes). 

http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm     icing ammounts