I'm going to use the Wilton Sports Ball Cake- it makes a 6" ball, so it's pretty small.   

But when I look at videos, it looks like a TOOOOOOON of extra fondant is needed to allow for the smoothing at the bottom.    Is there any sort of ratio?

Thanks!

I'm not sure if this is what you need, but Wilton has a chart for their own ready-made fondant: http://www.wilton.com/decorating/fondant/fondant-coverage-chart.cfm

Might be a place to start. :)

Thanks - That's a really good chart and I've bookmarked that for future reference...

unfortunately, these are all for flat pans.   I wonder if they have any for 3-D pans..... perhaps I'll call.