With regards to going forward with the project in the first place, you should ask yourself this:
If you are the only programmer on the project, then what is everyone else bringing to the table?
What is the revenue potential of the project?
Does your gut feeling tell you that it's going to succeed?
Is it something that you want to spend all those hours on?
If, after thinking about the above you still want to go forward with the project, then ask yourself these questions:
How good are you, and how hard would it be for your partners to replace you with someone else? If you are really good then ask for a high percentage, otherwise a lower one.
What is the minimum percentage that would make you interested? Have they said a number? Does it sound fair?
With regards to a good faith payment that should probably be based on what you need to survive. If this is just a sparetime project, then asking for an upfront payment will just lower the percentage you end up getting, so think about that too.