Remember the amount you need will depend on the fondant. For instance, I need less fondarific than satin ice.
When making a tall tier, I really on the total area I'll need to roll. My fondarific chart is based on 4" height. A 4" high 10" cake will require a total of 18" rolled fondant. A 6" high 6" cake will also require a total of 18" rolled fondant.