I was wondering if there is some way, other than guess work, of working out how much fondant is needed to cover a cake. I always end up colouring way too much.

Here is a fondant chart in which I use on occasion. Hope it helps.
http://www.earlenescakes.com/Fondantchart.html

I just bought the wilton book on fondant (not the course book) and in the back it has a chart that tells you how much to use on whatever size cake you have.

Thank you for your help.