I have a wedding cake that im making 14, 10, and 6 inch. (4"high) and im not sure how to know how much icing to make to just fill (adding mocha powder to it) and how much ill need to prep it for fondant. That wilton chart says about 15 1/2 cups but im thinking i might not need that much? It there an easy way to figure it out or another chart that im not seeing somewhere?

I'm trying to remember.....I think for the 6: I use 2 cups of batter, for the 8, 4 cups, for the 10 5 to 5.5, for the 12" 7 cups and I can't remember the 14"!!! aagghh.
One WASC recipe I usually count as 15 cups and for the fondant, 1 batch of Michelle Foster's fondant is enough to cover one 16" (with my mistakes)

I just recently used the Wilton chart for icing and I ended up having to make extra icing. I know it says to ice and decorate with, but I don't know if that includes what I used for the icing dam and the icing between the layers.

I always go with extra cause you can always freeze and re-use it later. When you have to make certain colors or flavors then it can be harder to match the colors later.