Advice needed on royalties for films

I'm going to be a puppet designer for a kids DVD soon, and I'm negotiating with the film maker on what my contract will be. I'm getting some money up-front to cover materials costs and labour, but I'm also going to be getting royalties if the DVD sells. What I'd like to know is what kind of percentage I should a) expect b) ask for as the sole puppet designer for the DVD. I've looked at MEAA, but haven't found anything related to these kinds of payments. Any help would be appreciated!