If 70% of the project is complete, and they have been accepted to an accelerator - then 10% is very fair, maybe even a little bit too fair. Your cofounder has

If 70% of the project is complete, and they have been accepted to an accelerator - then 10% is very fair, maybe even a little bit too fair. Your cofounder has created an immense amount of value already - seems to me you're getting a pretty good deal - grab it with both hands.