I'm pretty new to cake making and will be using fondant on a cake for the first time shortly. I want to plan out what to make with the least of amount of "left over" frosting ect.

How much buttercream do you use on the cake under the fondant?

I was looking at Wilton's cake making plans that give you the estimated plan for what size/serving for the cake shapes as well as batter amounts and frosting amounts. I assume that's for an all buttercream cake.

So when you add the fondant, do you use the same amount or less?

I am planning on two cakes, one a "figure 8" type cake and another as a sheet cake. I was thinking like 75% of the recommended amount?

The Wilton chart amounts account for icing and borders on the cake, so you would need probably need less just to crumb coat under fondant. As far as how much BC under fondant, that kind of depends. Some people are happy with just a very thin crumb coat to help the fondant stick to the cake, others want a thicker layer of BC so that you still get a nice amount of BC in every bite of cake.

I frost as I regularly would...as I don't really care for fondant and peel it away, then at least you have a choice!