[HELP] Contracts

I am in the process of a commercial project. Although before I begin I was required by the members to setup a contract of agreement to decide how payment would be decided. The way I figured, I would just take the effort put into the project and turn it into a percentage. You would then recieve that percentage of the profit when it was sold. The only problem is that we couldn't figure out how to really calculate effort. Now this isn't a game we are making its an engine script we hope to sell.

Has anyone worked on a project like this before, that could give me a sample overview of a contract. Does anyone have any advice? I want to get started on this code as quickly as possible but I cannot until I have an agreement in order. Any help is appreciated. Thanks.